A-Level Economics

Economics starts with one big idea: scarcity 稀缺性. We never have enough to give everyone everything they want.

• resources 资源 are the things we use to make goods and services. They are finite 有限的 — there is only a limited amount, so they are scarce 稀缺.
• goods 物品 (things you can touch, like food) and services 服务 (work done for you, like a haircut) are what we make from resources.
• human wants 欲望 are unlimited. People always want more.

Because wants are bigger than resources, this is called the economic problem 经济问题. Every society must answer three questions:

1. What to produce?
2. How to produce it?
3. For whom to produce it?

Choice and opportunity cost

Because resources are scarce, you must choose. Every choice means you give up something else.

opportunity cost 机会成本 — the next best alternative 替代选择 you give up when you make a decision.

Opportunity cost is faced by everyone:

• consumers 消费者 (people who buy and use goods): buy a phone, and you give up the holiday that money could have paid for.
• producers 生产者 and firms 企业 (the businesses that make goods): use a factory to make cars, and you give up the trucks it could have made.
• governments 政府: spend tax money on hospitals, and you give up the schools that money could have built.

Economics as a social science

Economics is a social science 社会科学. It studies how people behave when they make choices. Like other sciences, it builds models 模型 (simple pictures of the real world) and tests them with data.

The real world is complex, so economists change one thing at a time and keep everything else fixed. This is the idea of ceteris paribus 其他条件不变 — a Latin phrase meaning "all other things stay the same". For example: "if the price of coffee rises, people buy less coffee, ceteris paribus".

Positive and normative statements

• a positive statement 实证表述 is about fact. It can be tested as true or false. Example: "a higher price of petrol reduces the amount people buy."
• a normative statement 规范表述 is about opinion. It says what should happen, and it cannot be tested as true or false. Example: "the government should make petrol cheaper."

Normative statements rest on a value judgement 价值判断 — a personal view about what is good or fair. Two people can agree on the facts and still disagree about what should be done.

Microeconomics and macroeconomics

• microeconomics 微观经济学 studies single parts of the economy: one market, one firm, one consumer.
• macroeconomics 宏观经济学 studies the whole economy together: total output 产出, jobs, and rising prices.

The factors of production 生产要素 are the four kinds of resource used to make goods and services. Each one earns a reward 报酬.

Factor	What it is	Its reward
land 土地	natural resources (fields, water, oil, minerals)	rent 地租
labour 劳动	human work, both physical and mental	wages 工资
capital 资本	man-made things used to produce, like machines, tools and factories	interest 利息
enterprise 企业家才能	the skill of bringing the other three together and taking the risk	profit 利润

Land includes some non-renewable resources 不可再生资源 (like oil and coal) that can run out. Enterprise is supplied by the entrepreneur 企业家, the person who has the business idea and risks their own money.

Specialisation and division of labour

specialisation 专业化 is when a worker, firm or country makes only the things it is best at, then trades for the rest.

Inside a workplace, specialisation leads to the division of labour 劳动分工: a big job is split into small tasks, and each worker 工人 does one task again and again.

Advantages	Disadvantages
workers get faster and better at one task	doing one task is boring, so workers may quit
less time wasted moving between jobs	if one worker is absent, the whole line can stop
output per worker rises, so costs fall	workers learn only one narrow skill

Mobility of factors

mobility 流动性 means how easily a factor can move to a new use.

• occupational mobility 职业流动性 — how easily a factor moves from one job or use to another. A teacher who retrains as a nurse is occupationally mobile.
• geographical mobility 地理流动性 — how easily a factor moves from one place to another. Land cannot move at all, so it has no geographical mobility.

Labour is often not very mobile. Family ties, the cost of housing, and missing skills all make it hard for workers to move.

An economic system is the way a country decides how to allocate 配置 (share out) its scarce resources. There are three types.

• a market economy 市场经济: decentralised buyers and sellers influence production and allocation through prices and purchasing decisions.
• a planned economy 计划经济 (also called a command economy 指令经济): central decisions determine resource allocation and production, often using quantities and eligibility rules.
• a mixed economy 混合经济 — a blend of the two. The private sector 私营部门 (firms and households) and the public sector 公共部门 (the government) both make decisions. Almost every real country is mixed.

Worked allocation comparison. Three illustrative clinic arrangements provide the same kind of service. Private clinics responding to paying patients illustrate market allocation: prices, demand and expected costs influence what and how much to supply, and ability to pay affects access. A central authority specifying quantities, staffing and eligibility illustrates planning. Public tax-funded appointments alongside privately sold appointments illustrate a mixed arrangement. Public provision of one service does not make the whole economy planned. Planning and tax funding do not remove scarce staff time or the need to choose who receives appointments. Ownership alone does not prove efficiency or fairness.

The price mechanism

In a market economy, resources are guided by the price mechanism 价格机制. Prices do three jobs:

• signalling 信号传递 — a price tells buyers and sellers where resources are wanted. A rising price says "this is wanted — make more".
• incentive 激励 — a higher price rewards producers, so they want to supply more of that good.
• rationing 配给 — when a good is scarce, a higher price shares it out to those who are willing and able to pay.

Comparing the systems

System	Strengths	Weaknesses
market	choice, low prices, strong incentive to work and invent	large gap between rich and poor; some goods not supplied
planned	smaller gap between rich and poor; key goods provided for all	weak incentive; shortages and surpluses; slow to change
mixed	tries to keep the good points of both	the right balance is hard to find

In practice no country sits at a pure extreme. China's economy, for instance, is officially a socialist market economy, where markets set most prices while the government guides key sectors and owns major enterprises. Every real system mixes the two and sits somewhere on the spectrum below.

A production possibility curve 生产可能性曲线 (PPC) is a line showing the largest combinations of two goods an economy can make when all its resources are fully and well used.

Imagine an economy that makes only food and machines. The PPC is drawn with food on one axis and machines on the other.

• a point on the curve: production is efficient 效率 for the given resources and technology. This alone does not establish the mix society prefers.
• a point inside the curve: resources are wasted (for example, unemployment 失业 of workers). The economy could do better.
• a point outside the curve: impossible today, because there are not enough resources. This shows scarcity.

Worked frontier classification. An illustrative frontier joins food/machine combinations A=(20,0), B=(16,4), C=(10,8), D=(0,12), with food in tonnes/day and machines in units/day. E=(8,4) is inside: B permits more food at the same machine output. F=(15,8) is outside: at eight machines the maximum food is ten tonnes. C is efficient in production, but the frontier alone cannot show whether society prefers C to B. Moving from B to C gives up six tonnes/day for four extra machines/day, an interval average of 1.5 tonnes per machine. Unused capacity can permit more output without sacrificing the other plotted good; it does not make every use of scarce time, money or staff costless. Opportunity cost is not confined to points on a PPC.

Moving along the curve shows opportunity cost: to make more food, you must move resources away from machines, so you make fewer machines. This is the trade-off 取舍 between the two goods.

The shape of the curve shows how opportunity cost behaves:

• a straight-line PPC means constant opportunity cost — each extra unit of food costs the same number of machines.
• a curve that bows outwards means increasing opportunity cost — each extra unit of food costs more and more machines, because resources are not equally good at making both goods.

Shifts and growth

When the PPC moves outward, productive capacity increases: this is potential economic growth 经济增长. Actual output need not immediately rise to the new frontier. A sector-specific improvement can rotate one part of the frontier rather than shifting both intercepts. It happens when there are more resources, or better ones (new technology 技术, a bigger or more skilled workforce). War or disaster can move the PPC inward — this is economic decline 经济衰退.

A country must also choose between two kinds of good now:

• capital goods 资本品 — goods used to make other goods (machines, factories).
• consumer goods 消费品 — goods people use directly (food, clothes).

Making more capital goods today means fewer consumer goods today. Productive investment can increase future capacity, provided it adds effective resources after wear and replacement. More capital spending does not guarantee a larger shift: quality, use, skills and other constraints matter.

Worked example. Why is a production possibility curve usually drawn concave (bowed outwards) rather than as a straight line? A straight line would mean the opportunity cost of each extra unit stays constant. The curve bows out because factors of production are not perfectly substitutable - they are specialised. Starting from all-food production, the first factors moved into making cars are the ones least suited to farming, so very little food is given up for a lot of cars. Keep going and you must move factors that are increasingly good at farming and increasingly bad at making cars, so each extra car costs more and more food. That rising cost is increasing opportunity cost, and the bowed shape is simply its picture. A straight-line PPC is not wrong - it just assumes factors are perfectly substitutable, and naming that assumption is what earns the mark.

Free goods and economic goods

• a free good 免费物品 has no opportunity cost in the relevant use: available supply is sufficient without sacrificing another valued use. Freely available air may qualify in one setting; supplying clean air or oxygen in a hospital can require scarce resources. A zero charge alone does not prove a good is free in this economic sense.
• an economic good 经济物品 uses scarce resources and has opportunity cost. A private good 私人物品 has rivalry and excludability. An apple is an example; economic resources can also be used to provide public services. A good can have opportunity cost even if its user is not directly charged.

A private good has two features. It has rivalry 竞争性 (if I eat the apple, you cannot) and excludability 排他性 (the seller can stop you using it unless you pay).

Public goods

A public good 公共物品 is the opposite of a private good. It has two features:

• non-rivalry 非竞争性 — one person using it does not reduce the amount left for others.
• non-excludability 非排他性 — you cannot stop people who do not pay from using it.

Because of non-excludability, public goods suffer from the free-rider problem 搭便车问题: people enjoy the good without paying for it. This can cause private under-provision because voluntary payments may not cover cost. It does not prove nobody contributes: donations, sponsorship or other funding can support some provision. Government financing is one response; the financing body and the actual provider need not be the same. Street lighting and national defence 国防 are standard examples under the non-rival, non-excludable assumptions.

Worked classification. An uncrowded local street light benefits nearby users and cannot readily exclude non-payers, fitting the public-good conditions in that setting. A publicly funded hospital appointment uses a scarce doctor's time even when the patient pays nothing. It can be reserved for a patient and cannot simultaneously serve everyone, so public ownership does not make it a public good. A paid lesson can be private and merit at once if the learner underestimates its benefit; the information test differs from rivalry and exclusion.

Merit and demerit goods

• a merit good 优值品 is better for you (and for society) than you realise, so people buy too little of it. Examples: education and health care.
• a demerit good 劣值品 is worse for you than you realise, so people buy too much of it. Examples: cigarettes and alcohol.

People misjudge these goods because of information failure 信息失灵 — they do not have, or do not use, the full facts about the benefit or harm.

Deciding what counts as a merit or demerit good is partly a value judgement, so people may disagree about which goods belong on each list.

• Frame answers around scarcity and choice: every choice has an opportunity cost — the next best alternative given up.
• Read a PPC: a point inside means unused resources, on the curve is efficient, outside is unattainable; the slope shows opportunity cost.
• Distinguish positive (a testable fact) from normative ("should", a value judgement) statements.
• Classify goods: free versus economic, and public / merit / demerit goods (non-excludable, non-rival, and so on).

Vocabulary card meanings

goods: Physical items that people can buy and sell. alternative: Any other option a person could have chosen. social science: A field that studies how people behave in groups and markets. ceteris paribus: Holding every other factor unchanged while one factor is studied. positive statement: A claim that can be checked against facts. normative statement: A claim about what ought to happen, resting on a value judgement. microeconomics: The study of single markets, firms, and households. reward: The payment a factor of production receives for its use. rent: The payment made for the use of land. non-renewable resources: Inputs that nature does not restore within a human lifetime after they are used. division of labour: Splitting production so that each person does one task. worker: A person who supplies labour. mobility: How easily a factor can move to another job or place. geographical mobility: How easily a person can move to work in another place. planned economy: An economy in which the state decides what is produced. command economy: An economy run by government orders rather than by prices. economic decline: A fall in production and incomes across the whole economy. national defence: Protection of a country by its armed forces.

demand 需求 is the quantity of a good that buyers are willing and able to buy at each price 价格 in a period of time. It is not just a wish — buyers must also be able to pay.

The law of demand, holding other determinants constant, says: when the price rises, the quantity demanded 需求量 falls; when the price falls, the quantity demanded rises. Price and quantity move in opposite directions.

So the demand curve 需求曲线 slopes downward, from top-left to bottom-right.

What shifts the demand curve

A change in the good's own price is shown as a movement along the demand curve. A change in a non-price determinant shifts the whole curve to a new position — this is a shift 移动 of demand. The main causes (the determinants 决定因素) are:

• income 收入 — for most goods, higher income raises demand.
• the price of a substitute 替代品 — a good you can use instead (tea for coffee). If the substitute's price rises, demand for this good rises.
• the price of a complement 互补品 — a good used together with this one (cars and petrol). If the complement's price rises, demand for this good falls.
• tastes and fashion, the size of the population, and advertising.

Apply movement versus shift. Consider three separate coffee-market events. (1) Coffee price falls from $\$7$ to $\$6$, all other conditions fixed: quantity demanded rises along the original curve. (2) Tea becomes dearer: coffee demand shifts right if tea is a substitute. (3) Income rises, but no income relationship is given: the direction needs a normal/inferior-good assumption. Draw the relevant change before reading each explanation.

Individual and market demand

One consumer 消费者 has an individual demand. The market demand 市场需求 is found by adding up the quantity that all consumers want at each price (adding the curves sideways).

Marginal utility and demand

utility 效用 is the satisfaction you get from consuming a good. marginal utility 边际效用 is the extra utility from one more unit.

The law of diminishing marginal utility 边际效用递减 says each extra unit gives less extra satisfaction than the one before. This can help explain lower willingness to pay in a simple model, but it is not by itself a general proof of downward-sloping demand. Full utility analysis is an A-Level Topic 7 preview here. For marginal utilities $12,9,6,3$, total utility is $12,21,27,30$: total utility rises while marginal utility stays positive.

supply 供给 is the quantity of a good that producers 生产者 are willing and able to sell at each price in a period of time.

The law of supply, holding other determinants constant, says: when the price rises, the quantity supplied 供给量 rises. Higher prices can make extra output worthwhile. Total profit 利润 also depends on costs; price alone does not establish it. So the supply curve 供给曲线 slopes upward.

What shifts the supply curve

A change in the good's own price is a movement along the supply curve. Other causes shift the whole curve:

• costs of production 生产成本 — if wages, raw materials or rent get dearer, supply falls.
• technology — better machines raise supply.
• a per-unit producer subsidy 补贴 raises supply; a per-unit tax on sellers lowers it.
• the number of firms 企业 in the market, and weather (for farm goods).

Bakery application. A higher bread price, other conditions fixed, raises quantity supplied along the original curve. Cheaper flour or a more efficient oven shifts supply right: more bread can be offered at the same price. A change unrelated to willingness or ability to produce does not automatically shift supply.

Individual and market supply

The market supply 市场供给 is found by adding up the quantity all firms will sell at each price.

price elasticity of demand 需求价格弹性 (PED) measures how much the quantity demanded responds when the price changes.

PED is normally negative (because of the law of demand), but we usually look at the size and drop the minus sign.

Worked example. The price of a good rises by 10% and the quantity demanded falls by 25%. Find the PED and classify the demand.

Ignoring the minus sign, $2.5 > 1$, so demand is elastic — quantity responded much more than the price did.

PED magnitude	Name	Meaning
more than 1	elastic 富有弹性	quantity responds a lot
less than 1	inelastic 缺乏弹性	quantity responds a little
equal to 1	unit elastic 单位弹性	quantity changes by the same % as price
0	perfectly inelastic 完全无弹性	quantity does not change at all
infinity	perfectly elastic 完全有弹性	horizontal demand at a given price in the idealized model

What makes demand elastic or inelastic

• the number and closeness of substitutes (more substitutes → more elastic).
• whether the good is a necessity 必需品 or a luxury 奢侈品; these influence responsiveness but do not guarantee a particular own-price coefficient.
• the share of income spent on the good (a large share → more elastic).
• time — more time usually lets buyers find alternatives, making demand more responsive; it does not guarantee elasticity above 1.
• habit or addiction can reduce responsiveness, alongside other determinants.

One straight-line curve. For $Q_d=10-P$, the gradient $\Delta Q_d/\Delta P=-1$ is constant. Point elasticity 点弹性 uses that gradient with the current price and quantity:

At $(P,Q)=(8,2)$, $PED=(-1)(8/2)=-4$; at $(5,5)$, it is $-1$; at $(2,8)$, it is $-0.25$. The curve has the same slope but different percentage responsiveness. These are point values, not a claim that a large finite change has the same elasticity. Revenue $TR=PQ$ is $16,25,16$ respectively: the midpoint maximizes revenue on this straight-line curve with positive intercepts. A constant unit-elastic demand curve instead keeps revenue constant throughout its unit-elastic segment.

PED and total revenue

total revenue 总收益 is the money a firm earns: $\text{TR} = \text{price} \times \text{quantity}$. PED tells a firm what happens to revenue when it changes the price:

If demand is…	price rises	price falls
inelastic	total revenue rises	total revenue falls
elastic	total revenue falls	total revenue rises
unit-elastic segment	total revenue stays the same	total revenue stays the same

Worked revenue comparison. Both goods start at price $\$10$ and quantity $100$. Price rises to $\$11$ ($+10\%$, using the initial base). Good A sells $95$ ($-5\%$); good B sells $75$ ($-25\%$).

Higher revenue does not establish higher profit without cost information. Governments 政府 also use responsiveness when considering taxes, but tax revenue requires the tax per unit and the taxed quantity; PED alone gives neither.

Official application: 9708/21 O/N/25 Q2(a). Explain why a commuter may keep using petrol this week but change transport or car over a year. Then judge whether time is decisive: a remote commuter with no viable alternatives may still be unresponsive later. Use the formula, explain the adjustment mechanism and reach a conditional judgement; do not just list determinants.

income elasticity of demand 需求收入弹性 (YED) measures how much demand responds when income changes.

• a normal good 正常品 has positive YED: demand rises as income rises. Necessities have YED between 0 and 1; luxuries have YED above 1.
• an inferior good 低档品 has negative YED: demand falls as income rises (people switch to better goods). Cheap instant noodles may be inferior for some consumers over a particular income range; the label is not a judgement about quality.

Worked example. A person's income rises by 20% and their demand for restaurant meals rises by 30%. Find the YED and classify the good.

YED is positive and greater than 1, so restaurant meals are a normal good — and a luxury.

YED helps firms plan for income changes. The nature of the good, income level and whether alternatives are available can affect its value; the estimate need not remain fixed as conditions change.

cross elasticity of demand 需求交叉弹性 (XED) measures how much demand for one good responds when the price of another good changes.

• for substitutes, XED is positive (the price of B rises, so people buy more A).
• for complements, XED is negative (the price of B rises, so people buy less A and less B).
• for goods that are not related, XED is about zero.

Worked example. When the price of tea rises by 10%, the quantity of coffee demanded rises by 4%. Find the XED and say how the two goods are related.

XED is positive, so tea and coffee are substitutes.

XED depends on the closeness of substitutes or strength of complementary use, and on the market definition. A positive value predicts that a rival price cut reduces this good's demand, other determinants fixed.

Complement practice. Printer price rises $10\%$ and ink quantity demanded falls $6\%$: $XED_{ink,printer}=\frac{-6}{10}=-0.6$. The negative sign supports complementary use. It says nothing about whether ink is an inferior good; that needs an income change.

Decision practice for 9708/21 O/N/25 Q2(b). A fictional restaurant has YED $=1.5$ and XED with a close rival $=0.8$. In separate scenarios, income rises $2\%$ or the rival cuts price $10\%$.

YED matters for income forecasts; XED matters for rival-price changes. Use the expected changes and business decision to judge which is more useful. These are estimates holding other determinants fixed, not guaranteed exact forecasts or an unconditional ranking of the two measures.

price elasticity of supply 供给价格弹性 (PES) measures how much the quantity supplied responds when the price changes.

PES is nonnegative for an upward-sloping or vertical supply curve. A value above 1 is elastic; below 1 is inelastic. Compare percentage changes, not slope alone.

Compare two firms

Both start at price $10$, quantity $100$. Price rises to $12$ ($+20\%$). A supplies $110$ ($+10\%$); B supplies $140$ ($+40\%$). Percentages use the initial values.

Firm A is inelastic and firm B elastic. The diagram uses the same initial values and scales; visual steepness alone cannot classify arbitrary supply curves.

What makes supply more elastic:

• spare capacity 闲置产能 — unused machines and workers, so firms can quickly make more.
• stocks 库存 — goods stored in a warehouse that can be sold fast.
• how easily factors of production can move into the industry.
• time. Economists split time into three periods:
  • the momentary 瞬时 period — supply is fixed; PES is zero.
  • the short run 短期 — at least one factor is fixed; spare capacity and stocks may still allow a substantial response.
  • the long run 长期 — all factors can change, usually allowing more adjustment. This does not guarantee PES above 1.

Policy application: 9708/02/SP/23 Q3(b)

Refrigeration can preserve saleable stocks; training can ease skill constraints. Neither necessarily makes all agricultural supply elastic: perishability, costs and biological growing time differ. Explain the constraint removed, a limitation, then a product-specific judgement. A supply shift after a subsidy is not by itself evidence that PES has increased.

Compare the quantities demanded and supplied at the same price.

Equilibrium

The market is in equilibrium 均衡 where the demand curve crosses the supply curve. Here quantity demanded equals quantity supplied at the same price. This gives the equilibrium price 均衡价格 and the equilibrium quantity 均衡数量. There is no pressure for the price to change.

Worked market. Let $Q_d=12-P$ and $Q_s=P-2$. At prices $4,6,7,8$, the corresponding demand quantities are $8,6,5,4$ and supply quantities $2,4,5,6$. Solve the equality:

At $\$6$, excess demand is $6-4=2$ units; at $\$8$, excess supply is $6-4=2$ units. A shortage provides signalling 信号传递 of unmet demand. A higher price performs rationing 配给 through willingness and ability to pay and gives suppliers an incentive 激励 to offer more, other costs unchanged.

Disequilibrium

When the price is not at equilibrium, the market is in disequilibrium 非均衡:

• if the price is too low, there is excess demand 超额需求 (a shortage 短缺). Buyers compete, so the price is pushed up.
• if the price is too high, there is excess supply 超额供给. Sellers cannot sell everything, so the price is pushed down.

With other conditions unchanged and prices free to adjust, these imbalances create pressure towards equilibrium. Contracts, controls or new shocks can interrupt the path. This shows the price mechanism doing its jobs of signalling, rationing and giving an incentive.

How shifts change the equilibrium

Change	Effect on price	Effect on quantity
demand rises (shifts right)	rises	rises
demand falls (shifts left)	falls	falls
supply rises (shifts right)	falls	rises
supply falls (shifts left)	rises	falls

Demand and supply in different markets

The same analysis works in many markets:

• in product markets 产品市场, demand and supply set the price of goods and services.
• in factor markets 要素市场, they set the price of factors — for example, the demand for and supply of labour set the wage 工资.
• in a floating foreign exchange market 外汇市场, the demand for and supply of a currency set the exchange rate 汇率 (the price of one currency in terms of another).

Relationships between markets

• Joint demand 共同需求: printers and ink are complements; more printer use can increase ink demand.
• Alternative demand 替代需求: substitutes such as tea and coffee; dearer tea can increase coffee demand.
• Derived demand 派生需求: bread demand creates demand for bakers' labor.
• Joint supply 联合供给: beef production also produces hides; more beef output can increase leather supply.

Identify the market and relevant determinant before drawing its curve.

consumer surplus 消费者剩余 is the gain to buyers. It is the difference between the most a consumer was willing to pay and the price actually paid. On a diagram it is the area below the demand curve and above the price line.

producer surplus 生产者剩余 is the gain to sellers. It is the difference between the price a producer receives and the lowest price the producer would have accepted. On a diagram it is the area above the supply curve and below the price line.

Worked surplus and cost change. Use the same fictional competitive market: inverse demand $P_D=12-Q$ and inverse supply $P_S=2+Q$. Equilibrium is $Q=5$, $P=\$7$.

For 9708/22 O/N/25 Q2(a), consider the claim that producers always gain when a cost increase raises price. These are illustrative numbers, not data from that paper. Higher input costs shift supply to $P_{S,1}=4+Q$: Price rises, but producer surplus and revenue fall in this example. The new equilibrium can still be efficient: lower total surplus reflects the higher real production costs, not automatically deadweight loss. Judge the outcome using the new cost curve, quantity and demand/supply responsiveness. Producer surplus is not automatically total profit because fixed costs also matter.

A demand increase raises equilibrium price and quantity with supply fixed, but do not infer consumer surplus from price alone: both the demand curve and price have changed. A supply increase raises consumer surplus with demand fixed; producer surplus depends on the new curve and outcome. If a price is imposed away from equilibrium, specify the quantity actually traded and any rationing before measuring areas.

Consumer surplus plus producer surplus is total market welfare 福利 in this simple model. Competitive equilibrium maximizes it when private costs and benefits reflect social costs and benefits and there are no market failures. This is a conditional benchmark, not a claim about every real market.

• Quote the elasticity formula and interpret the value: PED magnitude inelastic ($<1$), elastic ($>1$); link PED to total revenue.
• Distinguish a movement along (a price change) from a shift (a non-price factor) of demand or supply.
• Use the sign of XED (substitutes $+$, complements $-$) and YED (normal $+$, inferior $-$).
• On a diagram, label consumer and producer surplus and show how a price change alters each.

Vocabulary card meanings

market demand: The total amount buyers want at each price. diminishing marginal utility: Each extra unit adds less satisfaction than the unit before it. market supply: The total amount sellers offer at each price. perfectly elastic: A situation in which price stays unchanged across the whole range of quantities buyers or sellers choose. momentary: Lasting only an instant, before anyone can adjust. equilibrium quantity: The amount bought and sold when buyers' and sellers' plans match. product market: The place where finished goods and services are traded. point elasticity: Responsiveness of quantity, measured at one exact price. joint demand: Demand for two products that are wanted together. alternative demand: Demand for two products that can replace each other. joint supply: Two products that come from the same production process.

A free market often gives a good result, but not always. market failure 市场失灵 is when the market does not allocate 配置 (share out) resources efficiently — too much or too little of a good is made. Then the government may step in.

The AS core in this topic is public goods, merit and demerit goods, price intervention, and income and wealth inequality. The externality diagrams and permit discussion below are a preview of the later A-Level microeconomics units; they do not replace the AS requirements.

Externalities

An externality 外部性 is a cost or a benefit that falls on a third party 第三方 — someone who is not the buyer or the seller.

• a negative externality 负外部性 is an external cost 外部成本 paid by others, like the pollution from a factory. The social cost 社会成本 (the cost to all of society) is bigger than the private cost 私人成本 (the cost to the producer). When people ignore the external cost, the market makes too much of the good.
• a positive externality 正外部性 is an external benefit 外部收益 enjoyed by others, like the protection that vaccination gives to everyone. Here the market makes too little, because buyers ignore the benefit to others.

Public goods

A public good 公共物品 has non-rivalry 非竞争性: one more user's benefit does not reduce what existing users receive. It also has non-excludability 非排他性: preventing non-payers from benefiting is impractical. A flood-warning broadcast to a whole valley is an example. An individual cinema seat is different: a ticket can exclude non-payers and an occupied seat is unavailable to another person. A school lesson can be a merit good without being a pure public good; places and teacher time are limited.

Worked financing problem. A warning service costs £90 for one period. Three households value it at £50, £40 and £30. Their combined benefit is £120, above its cost, but no household values it enough to pay the entire £90 alone. Once the warning is broadcast, each can benefit without paying. This free-rider problem 搭便车问题 can prevent an ordinary seller from recovering the cost. It is not proof that every voluntary contribution is impossible: agreement, donations or a larger organisation may fund some provision. Tax-funded provision can solve the charging problem, but government must still estimate benefits and costs and decide which service to fund.

Merit and demerit goods

A merit good 优值品 is consumed less than is considered desirable when people underestimate or overlook its benefits. A demerit good 劣值品 is consumed more than is considered desirable when people underestimate or overlook its harm. This information failure 信息失灵 concerns people's decisions about their own benefits and costs. A benefit or harm to a third party is a separate externality; one activity may involve both.

Compare two barriers. A training course charges £200. Some adults wrongly believe the qualification has no value; others understand its value but cannot pay. Information about recognised qualifications addresses the first barrier. A subsidy or free state place may address the second. If there are only 20 places, a campaign alone cannot create another 30 places. A good answer identifies the cause, explains the policy's effect on consumption, and checks resources and opportunity cost.

Monopoly power

A monopoly 垄断 is a single seller in a defined market. Monopoly power 垄断势力 is the ability to influence price rather than accept it. Restricting output can raise price and harm consumers, but costs, entry barriers and available substitutes matter. Formal efficiency and firm-behaviour analysis belongs to the later A-Level microeconomics units.

Inequality and asymmetric information

• differences in earnings and asset ownership can produce inequality 不平等. An unequal distribution and an inefficient allocation are different claims: inequality alone does not establish a failure of allocative efficiency.
• asymmetric information 信息不对称 means one party knows relevant information the other lacks. If a buyer cannot distinguish a reliable used car worth £6000 from a faulty one worth £2000, a cautious offer of £4000 may discourage reliable-car sellers. Inspection or a credible warranty may improve the deal. It is not inevitable that only faulty cars remain; detailed information problems are a later A-Level extension.

Indirect taxes

An indirect tax 间接税 is charged on goods or services. A specific tax is a fixed amount per unit; its amount does not rise with the selling price. In the competitive market model, it creates a wedge between the buyer's price and the amount the producer keeps. Keep these two prices separate.

Worked price wedge. Here P is pounds per unit and Q is units per period. Before tax, demand is P = 12 − Q and supply is P = 2 + Q:

A specific tax of £3 per unit changes the price buyers must pay for each supplied quantity to 5 + Q. The producer still keeps 2 + Q:

The incidence of a tax 税收归宿 is the distribution of the economic burden. Here buyers pay £1.5 more and producers keep £1.5 less per unit, so the burden is shared equally. The side legally sending the tax to government does not necessarily bear all of it.

Relative response matters. Holding supply conditions fixed, more inelastic 缺乏弹性 demand allows a larger buyer-price increase than more elastic 富有弹性 demand. When comparing who bears most, consider both demand and supply: the less responsive side bears more under the standard competitive model. Demand being inelastic alone does not prove that buyers bear most.

Counterexample: a different market. Demand is Qd = 120 − 2Pb and supply is Qs = 95 + 0.5Ps. The original price is £10 and quantity 100. At that point, absolute PED is 0.2 and PES is 0.05: demand is inelastic, but supply is even less responsive. A £2 tax gives Ps = Pb − 2:

Buyers bear 0.4/2 =20% of the tax; producers bear 1.6/2 =80%. This is a constructed counterexample, not an estimate for a real cigarette market.

Judge each policy aim separately. Raising revenue, reducing consumption and improving allocation are different outcomes. A small quantity fall can coexist with substantial revenue; it does not establish that the tax has no health benefit. For external costs, consider marginal harm and other conditions; not every tax moves output toward the social optimum. Whether its household burden is regressive also depends on spending and income patterns, not merely the name of the taxed good.

Subsidies

A subsidy 补贴 is a government payment; this example is a specific payment to producers for each unit sold. It creates a wedge in the opposite direction: producers receive the buyer's payment plus the subsidy. Effects depend on responses and market conditions; a fixed grant is not automatically a per-unit supply shift.

Worked producer subsidy. Before subsidy, equating 12 − Q and 2.5 + Q gives Q0 =4.75 and P0 =£7.25. With a subsidy £2.4 per unit, the buyer-facing supply price becomes 0.1 + Q:

Buyer price falls by £1.2 and producer receipts rise by £1.2 per unit; neither side receives the whole benefit of the subsidy in this model. Government spending is not the same as the increase in producers' receipts relative to the old price.

Merit-good decision. A subsidy may make a training course cheaper, but low awareness, travel costs or a shortage of teachers may still limit attendance. Compare a production subsidy with information about benefits and direct provision of places. Evaluate consumption achieved, who benefits and the opportunity cost of public funds. Do not conclude that any payment automatically corrects all underconsumption.

Maximum and minimum prices

A maximum price 最高限价 is a legal upper limit. It binds when below the relevant equilibrium; a higher ceiling may leave price and quantity unchanged. A minimum price 最低限价 is a lower limit, binding when above equilibrium. First test whether the control binds, then compare quantities demanded and supplied.

Ceiling case. In this separate market, Qd =10 − P and Qs =P. At a ceiling £3:

Without additional supply, only three units can be traded legally at that price. A shortage 短缺 is not a promise that every buyer gets cheaper goods. Queues, allocation rules or an illegal resale market may result. A ceiling £6 is above equilibrium and does not bind.

Floor case. At a floor £7, Qs =7 but Qd =3, creating excess supply 超额供给 of four units. Without government purchases, private buyers take only three units. If government guarantees to buy the remaining four at the floor, its spending is 7(4) =£28 per period and it must store or dispose of those units. A floor £4 does not bind. Changing the legal price is not the same as guaranteeing sales or household income.

Buffer stocks

A buffer stock 缓冲库存 scheme buys and stores a storable commodity when its price reaches a lower intervention price, then sells stock when price reaches the upper intervention price. Holding a band requires enough money and storage to buy, and enough usable stock to sell. It is not an automatic guarantee.

Worked harvest decisions. Demand is P =10 − Q. Normal supply P =Q gives equilibrium P5/Q5. The government targets a band £4.5–£5.5. An opening store contains three units.

Harvest	Farm supply	Intervention price	Qd	Farm Qs
Bumper	P=Q−2	£4.5 floor	5.5	6.5
Poor	P=Q+2	£5.5 ceiling	4.5	3.5

With the bumper crop, free-market price would be £4. At the target floor, buy 6.5 −5.5 =1 unit for £4.5; stock rises 3→4. With the poor crop, free-market price would be £6. At the target ceiling, sell 4.5 −3.5 =1 unit for £5.5; stock falls 4→3. These numbers exclude storage costs and spoilage.

If three poor-harvest periods start with stock 3 and no purchases, one-unit releases empty the store. In a fourth poor period the scheme cannot supply the missing unit without another source. Perishable goods, changing long-run demand and persistent surpluses create further problems.

Price stability is not identical to income stability. Farmers' receipts here are 4.5(6.5) =£29.25 after a bumper harvest, but 5.5(3.5) =£19.25 after a poor harvest: their own output differs. Selling stored stock does not give farmers the receipts from the current stock release. Compare buffer stocks with diversification, subsidies or other support when evaluating farm income.

Information and direct provision

State provision 政府提供 means that the government 政府 supplies or arranges a service. The service need not be free, and public funding does not by itself make it a public good. A free clinic still uses scarce appointments, staff and medicines. Direct provision can add capacity where a subsidy would only raise demand for fixed places; it needs finance, suitable staff and access for the intended users.

Choose an intervention. A district has 100 eligible children but only 60 childcare places. Forty additional places cost £4000 per period. A £1000 information campaign may help parents find existing vacancies, but it cannot remove this capacity shortfall if all 60 places are full. Compare the additional places with a different use of the same £4000, and check quality and location. Free childcare may enable parents to work; its total benefit is not measured only by the fee saved.

Later A-Level extension: regulation and permits

Regulation 管制 uses rules such as safety standards or pollution limits. Tradable permits 可交易许可证 allow firms to trade permission to emit under an enforced cap. Trading can move reductions towards firms able to cut pollution cheaply, but this depends on monitoring, enforcement and a functioning market; it does not guarantee minimum costs under all conditions. For example, if A can cut one unit for £10 and B for £30, making A cut meets a one-unit reduction target at lower resource cost. A trade can reward that cheaper reduction. The cap and compliance must still be checked.

Evaluating intervention and government failure

A policy being costly is not enough to show government failure 政府失灵. Compare its benefits and full costs with the outcome without the policy and with a relevant alternative. Failure means intervention makes resource allocation worse. Poor information, administration costs, unintended effects or decisions serving a narrow group can cause this. A binding ceiling can encourage an illegal black market 黑市, but that is a risk to analyse, not an automatic result of every price control. Formal government-failure analysis is developed in the later A-Level unit.

Illustrative comparison. A programme produces benefits worth £120 and uses resources costing £100, giving a net gain of £20. It is not a failure just because it costs £100. If another feasible programme gives benefits £140 for the same cost and the same relevant risks, its net gain is £40. Identify the objective, evidence and alternative before judging.

Income 收入 is a flow 流量 received over a period, such as wages, rent and interest. Wealth 财富 is a stock 存量 of assets owned at a point in time; net wealth subtracts debts. A person receiving £2000 each month does not therefore own £2000 of wealth. A house worth £150000 with a mortgage of £100000 contributes £50000 to net wealth. Savings can build wealth, and assets can produce income, but these are different measures.

Measuring and explaining inequality

The Gini coefficient 基尼系数 summarises inequality on a scale from 0 (complete equality) to 1 (the complete-inequality limit). Specify whether the data measure income or wealth, and use a comparable population, period and definition. The AS syllabus requires interpretation, not calculation of the coefficient.

Worked interpretation. A fictional country's disposable-income Gini falls from 0.42 to 0.35. Its measured income distribution becomes more equal. This does not establish that every poor household receives more money, that average real income rises, that wealth becomes more equal, or that one policy caused the change. Government can use the measure to review redistribution, alongside household-group data, employment, prices and access to services. Two countries with the same Gini may have different average living standards and different distributions within income groups.

Income differences may reflect qualifications, experience, demand for occupations, hours worked, unemployment, discrimination or bargaining power. Wealth differences may reflect accumulated savings, inheritance, asset-price changes and debts. Causes interact: unequal access to education may affect future earnings, and inherited assets may generate rent. Do not rank one cause as universally most important without evidence.

Progressive taxes and transfer payments

A progressive tax 累进税 takes a higher average percentage 百分比 of higher incomes. Compare average rates, not just amounts paid. Benefits 福利金, also called transfer payments 转移支付, are government payments without a current good or service received in return; eligibility differs between programmes.

Worked redistribution. Two households have annual incomes £10000 and £40000. Taxes of £1000 and £8000 give average rates 10% and 20%. If the lower-income household receives a £2000 transfer, their disposable incomes are:

The income ratio falls from 4 to about 2.91. This is a two-household illustration, not a calculation of a national Gini or a claim that these taxes alone finance the entire government. Equal £1000 taxes instead take 10% and 2.5% of the original incomes; paying more pounds is not the definition of progressivity.

Minimum wages and access to essential services

A minimum wage is a legal floor on hourly pay. In an illustrative competitive labour market, demand is Ld =100 −5w and supply is Ls =5w. The original hourly wage is £10 and employment 50 workers. A binding £12 floor gives demand 40 and supply 60: 40 jobs and excess labour supply of 20 under this model. Workers who keep the same hours earn more, but excluded workers do not receive that wage. Household effects also depend on hours, other earners, prices and benefits. A floor £8 does not bind here. Other labour-market structures and their full analysis are developed later; the competitive prediction is not a universal empirical law.

Free or subsidised essential services can raise real living standards without increasing cash income. A £500 saving on childcare can matter to a household even if its measured cash income is unchanged. Funding, quality, waiting times and location determine whether poorer households can actually use the service. Education and childcare access may also improve future earnings; redistribution need not always reduce efficiency.

Income taxes compared with inheritance and capital taxes

Inheritance taxes target transfers of assets after a death; capital taxes target specified assets, ownership or gains, depending on the policy. They can address accumulated wealth that an income tax misses. Accurate valuation, debt, exemptions, collection and avoidance matter. A valuable house may produce little cash for paying a tax, while assets with reliable ownership records may be easier to assess than hidden business holdings.

Policy judgement. Regular recorded wages can make income taxation and transfers easier to administer than taxes on hard-to-value wealth. This is a conditional argument, not a rule that wealth can never be redistributed. Land records or estate information can assist wealth taxation; informal earnings can make income taxation difficult too. Assess which inequality is targeted, administrative capacity, avoidance and the effects on work, investment and opportunity. This redistribution 再分配 can involve a trade-off 取舍 when marginal tax and benefit rules weaken an incentive 激励 to earn more, but the strength of that response needs evidence.

• Identify the actual problem: public-good financing, misjudged benefit or harm, a price objective, or income/wealth inequality.
• Explain the mechanism with a relevant example, diagram or calculation. Distinguish prices, quantities, fiscal costs and household outcomes.
• Evaluate against the stated aim and a feasible alternative. Give a reasoned conclusion using conditions, not a fixed list of advantages and disadvantages.
• In tax incidence compare both demand and supply responses. For Gini data explain what the measure can and cannot establish; no Gini calculation is required.
• The externality diagrams, monopoly discussion and pollution permits in this handout are later A-Level previews. Study the AS core requirements first.

Vocabulary card meanings

private cost: A cost paid by the person or firm that causes it. external benefit: A gain received by a third party who is outside the deal between buyer and seller. buffer stock: A store of a commodity kept to steady its price. black market: Trade that breaks a legal rule on price or quantity. wealth: The value of what a person owns after subtracting what that person owes, measured at one moment. stock: A quantity measured at one moment, not over a period of time. benefits: Payments from the state to people who qualify for help.

National income 国民收入 describes income generated by production during a period. GDP, GNI and NNI are related measures, but answer different questions. State the period, currency and price basis when comparing them.

Count final output or value added

Gross domestic product 国内生产总值 (GDP) is the value of final goods and services produced within an economy during a period. It includes government services, not just goods sold by private firms. Intermediate inputs are used to produce other output; do not count them again alongside that final output.

Value added 增加值 is output value minus the value of intermediate inputs bought from other producers.

Worked example — one domestic bread chain. All amounts are £ per batch. Assume no other bought inputs or product taxes.

Producer	Sales	Bought inputs	Value added
Farmer	20	0	20
Miller	35	20	15
Baker	60	35	25

The farmer sells grain, the miller sells flour and the baker sells the final bread to households. Adding sales gives £115, but counts the grain and flour more than once. Adding value added gives £60, the same as final bread sales.

Domestic, national, gross and net

Gross national income 国民总收入 (GNI) is residents' total primary income. Start with GDP, add primary income residents receive from abroad and subtract primary income paid to non-residents. Residency is different from citizenship. Export receipts are payments for output, not primary income from abroad.

Worked example — annual accounts. GDP is £800 billion/year, primary income received from abroad is £50 billion/year and income paid abroad is £70 billion/year, all at the same price basis.

A foreign-owned factory's domestic output counts in GDP. Income it pays to non-residents enters the GNI adjustment; we do not remove its entire output simply because its owners live abroad.

Capital depreciation 资本折旧 means consumption of fixed capital through wear and obsolescence. Net national income 国民净收入 (NNI) deducts this from GNI. If capital depreciation is £60 billion/year:

Gross/net adjusts for capital used up. It does not remove inflation.

Market prices and basic prices

Market prices 市场价格 include taxes on products less subsidies on products. Basic prices 基本价格 remove this net product-tax component. For the syllabus conversion:

Worked example — separate accounts. GDP at market prices is £500 billion/year, product taxes £80 billion/year and product subsidies £20 billion/year.

Reverse the signs to check the original market-price value:

Do not subtract personal income tax or capital depreciation in this price-basis step.

Nominal, real and per capita

Nominal 名义 values use current prices. Real 实际 values remove price changes so that changes in the amount of output can be compared. Nominal figures remain useful for current money receipts and payments.

Worked example — separate two-year series. Nominal GDP is £800 billion/year in Year 0 and £924 billion/year in Year 1. The given GDP price index rises from 100 to 110, with Year 0 as the base. Use a GDP price index, rather than assuming a consumer price index measures all GDP prices.

Per capita 人均 means per person. If population rises from 40 million to 42 million in this example:

Output and population both rise 5%, leaving real output per person unchanged. The average does not show who receives income.

A lower positive growth rate does not mean falling output. If a real output index starts at 100, rises 4% and then 2%, its levels are 104 and 106.08. Growth slows by 2 percentage points but output rises in both years. Distinguish a forecast from an observed outcome.

Uses and limitations

National income statistics help compare output over time and resources available for consumption and public services. For comparisons across countries, check prices, currency conversion, purchasing power, population and data quality.

The standard of living 生活水平 is wider than output per person. Consider distribution 分配, leisure, health, access to services and pollution. For example, two economies can have the same average but give the poorest fifth 10% and 3% of income respectively. The average alone cannot rank their living conditions.

Most unpaid household services, such as cleaning your own home, are outside GDP; paying someone to provide the service can count. The informal economy 非正规经济 includes activity outside or insufficiently covered by formal arrangements. Production within the GDP boundary should be included in principle, but it can be difficult to measure fully. Do not claim that all informal production is excluded because it is untaxed.

The circular flow of income 收入循环流 links production, income and expenditure. Households 家庭 supply factor services to firms 企业 and receive wages, rent, interest and profit. Households buy goods and services from firms.

Transaction	Real flow	Money flow
Employment	Labour to firms	Wages to households
Bread purchase	Bread to households	Spending to firms

In each transaction the real flow and its payment go in opposite directions. Saving means households need not spend all current income on consumption.

Leakages and injections

Leakages 漏出 are saving 储蓄 ($S$), taxation 税收 ($T$) and imports 进口 ($M$). Injections 注入 are investment 投资 ($I$), government spending 政府支出 on goods and services ($G$), and exports 出口 ($X$).

A newly produced factory machine is investment; buying an existing share is an asset transaction, not new capital production. Government purchases of teaching services count in G. A pension is a transfer payment, rather than payment for current output; the recipient's later purchases may enter consumption. Imported shoes enter household consumption and imports: subtracting M prevents foreign production from being counted as domestic GDP.

A closed economy 封闭经济 has no foreign trade, so X and M are zero. An open economy 开放经济 includes international transactions. Government can be present in either.

Equilibrium and disequilibrium

Equilibrium 均衡 in this simple model requires planned injections to equal planned leakages at the current income level. Actual expenditure accounts include unplanned inventory changes; do not mistake an accounting identity for proof that plans are in balance.

Worked example — annual planned flows at fixed prices. Income Y is £300 billion/year; S = 35, T = 25, M = 30, I = 30, G = 40 and X = 20, all in £ billion/year. Assume no transfer payments. Find consumption C:

Planned expenditure equals current output, so there is no unplanned inventory change.

Change the plan. Investment rises to 40 while current output and other planned flows initially stay fixed.

Unexpected inventory falls give firms a reason to raise production and employment if spare capacity allows. Near capacity, extra demand may mainly raise prices. As income changes, saving, taxes and imports may also change. An initial excess of injections does not guarantee a particular rise in real output. If planned spending falls below output, inventories rise unexpectedly and firms may reduce production. No multiplier calculation is needed here.

Check yourself

New annual data, all £ billion at the same price basis: GDP at market prices 600; product taxes 90; product subsidies 10; primary income received 30 and paid 50; capital depreciation 40. Find GDP at basic prices and NNI at market prices. Explain why neither answer alone establishes real growth.

Worked answer. The price-basis adjustment is:

The resident-income and capital-use adjustments are:

Real growth needs comparable real output for at least two periods. These are different measures for one period.

Aggregate demand

Aggregate demand 总需求 (AD) is planned spending on domestic goods and services at each price level. Its components are household consumption 消费 C, spending on new capital I, government purchases of goods and services G, and net exports 净出口 X − M.

Worked example — stated price level. Planned annual spending is C = 150, I = 30, G = 40, X = 25 and M = 35, all £ billion/year.

Net exports are negative: imports can exceed exports. Imported products can appear in C, I or G, so subtract M to remove foreign production. Buying an existing share is not spending on newly produced capital, and a pension is not a government purchase of current output.

A movement along AD and a shift

Hold other influences constant. A lower domestic price level can raise the purchasing power of money balances and make domestic goods cheaper relative to foreign goods. Real output demanded rises along the existing AD curve. This helps explain its downward slope.

By contrast, higher household confidence can increase consumption at each price level, shifting AD right. A change in a non-price influence changes the relationship, rather than moving along it.

Explain the decisions. A tax cut raises disposable income and may raise C, but households may save or repay debt; some purchases may be imports. Higher expected sales may encourage firms to buy new machinery, raising I, but spare existing capacity, expensive credit or uncertainty can weaken the response. Higher government purchases directly raise G. Stronger foreign demand may raise X. Explain the component and mechanism before drawing an AD shift; do not assume every change in income or borrowing causes the same spending response.

Aggregate supply

Aggregate supply 总供给 (AS) relates the price level to real output supplied. Short-run aggregate supply 短期总供给 (SRAS) slopes upward when some input costs adjust slowly. If output prices rise while wage rates and input prices initially stay fixed, producing extra output becomes more attractive: move up the same SRAS.

Higher imported fuel costs reduce profitable supply at each price level and shift SRAS left/up. Better productivity 生产率 can reduce unit costs and shift SRAS right. Separate a change in the price level from a change in a supply determinant.

Macroeconomic equilibrium

Macroeconomic equilibrium 宏观经济均衡 is the relevant AD/AS intersection. It determines the price level 价格水平 and real output 实际产出 in that model.

Compare changes in demand and supply

The paired diagrams below use a separate illustrative short-run model with upward-sloping SRAS and room for output to change. Each starts at the same initial intersection.

• When AD shifts right along upward-sloping SRAS, real output and the price level rise. Firms may need more workers; the employment response also depends on productivity and hours worked.
• When SRAS shifts left with AD unchanged, the price level rises while real output falls. Firms may reduce employment as they reduce production. This is a different mechanism from higher spending.

A shift showing a higher price level illustrates inflation pressure between periods. Persistent inflation means prices keep rising over time; one shock alone does not establish a permanently continuing inflation rate.

Capacity and the long run

Long-run aggregate supply 长期总供给 (LRAS) depends on productive capacity 生产能力: resources, skills, technology and productivity. More reliable electricity can reduce factory outages and expand sustainable production. Extra household spending initially shifts AD. Some spending, such as infrastructure construction, can raise demand now and improve supply later.

The syllabus permits vertical LRAS or a three-section representation: a highly elastic/flat section with spare capacity, an upward section as constraints increase, and a vertical section at capacity.

In the upward section, both output and prices can rise. Identify the model, the capacity position and the time period before concluding that every AD or AS shift changes both variables. Extra capacity does not guarantee enough demand to use it. These conditions explain why an increase in AD does not always lead to inflation.

Economic growth 经济增长 is an increase in real output over time. Actual growth 实际增长 is the observed increase. Potential growth 潜在增长 expands what the economy can sustainably produce.

Compare two changes. Restarting idle factories raises current output using existing capacity. Better machinery can expand capacity and shift LRAS right, even before firms receive more orders. Actual growth can occur without new capacity; potential growth can occur before actual output rises.

A production possibility curve 生产可能性曲线 (PPC) shows feasible output combinations using available resources and technology.

Causes of growth

More working-age residents can increase labour input; training can reduce defects and raise useful output per worker. Investment can improve capital, while technology and infrastructure can increase productivity. These mechanisms require suitable resources and demand: an unused machine does not automatically increase actual output.

For real output per person to rise, real output must grow faster than population. The earlier example shows why 5% real GDP growth alongside 5% population growth leaves the average unchanged. Distribution also affects who gains.

The business cycle

Real output does not grow smoothly. The business cycle 经济周期 includes boom 繁荣, recession 衰退, slump 萧条 and recovery 复苏. A recession involves falling real output; a recovery involves rising output after a downturn. Definitions using a particular quarterly rule require suitable quarterly data.

Worked example — separate annual output series. The real-output index is 104 in Year 1, 102 in Year 2 and 103 in Year 3.

Output falls and then rises, but 103 is still below 104. Recovery does not mean the previous output level has already been regained. A slower positive growth rate is also different from negative growth.

Costs and benefits of growth

Higher output can support incomes, jobs, tax receipts and public services. But examine employment, real output per person and distribution before assuming that everyone benefits equally or poverty falls.

Compare two illustrative paths: A reopens idle low-pollution factories; B produces more through longer working hours and more polluting activity. Both can raise GDP, but B may lose leisure and face health costs or resource depletion. Growth caused by extra demand near capacity can create inflation pressure; growth expanding supply can ease pressure. Judge the mechanism, scale, sustainability and who gains or loses.

unemployment 失业 means having no job while available for work and actively seeking it. The labour force 劳动力 is employed people plus unemployed people. People neither working nor actively seeking work are outside it; they are economically inactive. Classify students and retired people by their actual work and search status, not their label alone.

Measuring unemployment

The claimant count 申领救济人数 counts people claiming specified unemployment benefits. Administrative records are relatively cheap to collect, but miss non-claimants and ineligible jobseekers. Changes in eligibility can change the count without a change in jobs.

The labour force survey 劳动力调查 asks a representative sample about work, availability and active search. A common international definition helps comparisons, but sampling error, inaccurate replies and differences in implementation affect estimates. Discouraged people who stop searching are outside measured unemployment. A person working fewer hours than they want can still be classified as employed.

Worked example: choose the denominator. An illustrative population has 1000 people: 540 employed, 60 without jobs but available and seeking work, 150 inactive working-age people and 250 children. Let employed people be $E$ and unemployed people be $U$.

The unemployment rate measures the unemployed share of the labour force, not the whole population:

Dividing by 1000 would give $6\%$, the unemployed share of the total population, which is a different measure.

Worked example: a rate can fall without new jobs. Forty of these unemployed people stop seeking work; nobody gains or loses a job. Unemployment becomes 20 and the labour force becomes 560.

Employment stays at 540. Read employment and participation alongside the unemployment rate before concluding that jobs have improved.

Types and causes

• frictional unemployment 摩擦性失业: matching a worker to a vacancy takes time, for example while searching between jobs. It is often brief, but its duration is not fixed.
• structural unemployment 结构性失业: changes in production and demand leave workers' skills or locations mismatched with available jobs, for example after a local industry declines.
• cyclical unemployment 周期性失业: weak AD reduces sales and firms' demand for labour during a downturn. Its size and duration depend on the downturn and recovery.
• seasonal unemployment 季节性失业: predictable seasonal changes remove jobs at certain times, for example winter layoffs at a summer resort.
• technological unemployment 技术性失业: machines or software replace particular labour tasks. It overlaps with structural unemployment when displaced workers lack the skills needed for new jobs. Automation can also create jobs and reduce costs elsewhere; the displaced workers do not necessarily gain those jobs immediately. Do not count overlapping causes twice.

Consequences and comparison

Lost earnings can reduce household spending and living standards. Long spells without work can harm skills and health; savings, benefits and access to another job affect the harm. Unused willing labour means forgone output, and falling household spending can weaken firms' sales further. Production may lie inside the PPC when usable resources are idle, although jobseekers cannot necessarily fill every vacancy immediately.

Lower taxable earnings reduce government receipts; eligible benefit claims raise spending. The effect depends on tax and benefit rules, numbers affected and duration.

Worked comparison. In illustrative case A, 2000 workers lose jobs after a local industry closes and nearby vacancies need different skills. In B, 20000 workers lose jobs across industries during a demand downturn. A may cause persistent regional harm if retraining and moving are difficult. B affects ten times as many people and may cause a larger immediate loss of income and output. Neither type is always worse: compare numbers, duration, regional concentration, lost income and routes back to work. Without these data, a definite ranking is unjustified. Stronger AD helps weak sales; training helps a mismatch, but neither guarantees a suitable job. Unit 5 develops the policy analysis.

Inflation, deflation and disinflation

inflation 通货膨胀 is a sustained rise in the general price level. Money's purchasing power 购买力 falls: a given sum buys less. deflation 通货紧缩 is a sustained fall in the general price level. disinflation 反通货膨胀 means inflation slows while remaining positive: prices still rise, more slowly. A fall in one product's price does not establish economy-wide deflation.

Building and interpreting a CPI

The consumer price index 消费者价格指数 (CPI) tracks prices of a household consumption basket against a base period. A weight 权重 reflects a category's share of household spending, not the number of products. In this simplified calculation, category price indices use the same base year and weights sum to 1:

Worked example: a two-category basket. Food has weight $0.60$ and price index $120$; transport has weight $0.40$ and index $110$.

The simple average, 115, incorrectly gives equal weight to unequal spending shares. With a base-year index of 100, this basket costs $16\%$ more than in the base year.

Next year, food's index is 126 and transport's is 121. Keep the same weights:

The inflation rate is the percentage change from the previous index:

The index rose 8 points, but prices rose about $6.90\%$, not $8\%$.

Worked example: disinflation. A separate annual CPI series is 100, 104, then 107.

The rate falls, but the price level still rises: disinflation, not deflation.

Difficulties in measurement

An average basket does not represent every household. Using food index 120 and transport index 110, a household spending $80\%$ on food and $20\%$ on transport has a different cost index:

Its basket costs $18\%$ more than in the common base year, compared with $16\%$ for the official example. Basket updates can lag changes in buying habits. Quality changes, new products and representative price sampling also make like-for-like measurement difficult.

Nominal and real pay

Worked example. Weekly nominal pay rises from $\text{\pounds}500$ to $\text{\pounds}520$ while the CPI rises from 100 to 104. Express real pay in base-year pounds:

Initial real pay was also £500 per week. The pay rise buys the same basket. Wider living standards also depend on working hours, other income, household needs and public services.

Practice with solution. Pay rises from £500 to £550; CPI rises from 100 to 105. Find real pay and its percentage change before reading on.

Subtracting nominal and price growth rates gives an approximation ($10\%-5\%=5\%$), not the exact change.

Causes of inflation

demand-pull inflation 需求拉动型通货膨胀 comes from stronger planned spending shifting AD right. Price pressure depends on spare capacity and the supply response; an AD increase need not cause a large price rise. Investment can raise AD now and productive capacity later.

cost-push inflation 成本推动型通货膨胀 comes from higher input costs shifting SRAS left. With AD unchanged and upward-sloping SRAS, the new equilibrium has a higher price level and lower real output.

A one-off fuel-cost increase can raise the price level without causing repeated annual price rises. Continuing inflation requires continuing pressure or further adjustments. State the time period and AS conditions rather than treating every increase in spending or costs as continuing inflation.

Consequences of inflation

For consumers, income that fails to keep pace loses purchasing power. The effect varies with spending patterns, wage adjustment and savings. High, unpredictable inflation can make planning harder and weaken confidence.

Worked example: fixed nominal debt. A debt stays at £1000 while the price index rises from 100 to 120.

Its real value falls from £1000 in base-year pounds. Unexpected inflation can favour a fixed-rate borrower over the lender. Indexed debts or higher interest charges can offset this; repayment ability depends on income. Savers' real returns depend on the interest earned relative to inflation. Borrowers do not always gain and savers do not always lose.

For firms, higher prices do not guarantee higher profit. Demand-led inflation may support sales, whereas cost-push inflation may squeeze margins. Export competitiveness depends on foreign prices and exchange rates as well as domestic inflation.

Worked example: price, cost and sales. An illustrative firm has no other costs. Price starts at £10 per unit, unit cost £8 and sales 100 units. Later price is £12, unit cost £11 and sales 80 units.

Selling prices rise but profit falls because margins and sales fall. High inflation also makes frequent repricing costly; uncertain costs and demand can discourage investment and make price signals harder to interpret.

Assessment practice. Is high inflation always more damaging to consumers than to firms? Compare a fixed-income household buying essentials with a firm using imported inputs that cannot quickly raise selling prices. Explain each mechanism, then judge using income adjustment, debt terms, spending patterns, market power, input exposure and duration. Keep the answer about high inflation if that is the question's context. Neither group is always harmed more.

Why deflation's cause matters

A collapse in AD can lower both prices and output, increasing unemployment. Expectations of further price falls may delay some purchases, especially durable goods, weakening demand further. Fixed nominal debts become more burdensome in real terms. But productivity improvements can lower prices while raising output. Falling prices alone do not prove a demand collapse: cause, expectations and debt exposure determine the risks.

1. Nominal GDP rises $8\%$ while the GDP price index rises $10\%$. Does real output rise? Explain rather than subtracting rates as an exact calculation.
2. The unemployment rate falls with employment unchanged. Explain a possible cause.
3. Factories have spare capacity. Why need higher investment not cause a large immediate price rise? Name a leakage and distinguish it from an injection.
4. CPI rises from 125 to 130; weekly pay rises £600 to £612. Calculate inflation and the real-pay change.
5. Automation displaces packers while technician vacancies remain. Explain the unemployment type and evidence needed to compare the harm with a demand downturn.

Check after attempting. (1) Real-output factor $1.08/1.10\approx0.982$: real GDP falls about $1.82\%$. (2) Some jobseekers may leave the labour force. (3) Spare capacity allows more production with limited price pressure; later investment may expand supply. Saving, taxation and imports are leakages; investment, government spending and exports are injections. (4) Inflation is $[(130-125)/125]\times100\%=4\%$. Real-pay change is $[(612/600)/(130/125)-1]\times100\%\approx-1.92\%$. (5) Technological displacement can overlap with structural skill mismatch; compare numbers, duration, lost income and access to suitable jobs before ranking harm against cyclical unemployment.

Vocabulary card meanings

gross national income: Total income earned by a country's residents, at home and abroad. real: Adjusted so that rising prices do not inflate the figure. standard of living: The material comfort people can afford from their income. distribution: How income or output is shared among people. leakages: Income that leaves the circular flow through saving, tax, or imports. closed economy: An economy that does not trade with other countries. open economy: An economy that trades goods and capital with other countries. productive capacity: The most an economy can make with the resources it has now. real output: Production counted in constant prices, so inflation is removed. slump: A deep, long fall in production and jobs. purchasing power: How much a given sum of money can buy. cost-push inflation: Rising prices caused by higher costs of making goods. value added: The extra worth a firm creates beyond the inputs it buys. net national income: Residents' income after wear and tear on capital is subtracted. capital depreciation: The loss of value in machines and buildings as they wear out. market prices: The prices buyers actually pay, including indirect tax. basic prices: The prices producers receive before indirect tax is added. technological unemployment: Job loss because a machine now does the work.

The core AS objectives in this unit are price stability 物价稳定, low unemployment and economic growth 经济增长. Price stability means low, stable inflation 通货膨胀; use the target stated in a question rather than assuming every country has the same numerical target. Growth means rising real output. Check population and distribution before claiming higher output improves everyone's living standards.

Low unemployment means people seeking and available for work can find suitable jobs. full employment 充分就业 does not require zero measured unemployment: workers and vacancies take time to match. Read employment and participation alongside the unemployment rate.

Fiscal and monetary tools are often called demand-side policies 需求侧政策 because they influence aggregate demand 总需求. But policy labels can overlap: the design can also affect aggregate supply 总供给.

Worked explanation. Government purchases of port construction immediately raise $G$, a component of AD. If the finished port reduces transport delays, it can later raise productive capacity. Fiscal spending can therefore have a demand effect now and a supply effect later; the second effect depends on the project's quality and use.

Broader objectives include a sustainable current-account position within the balance of payments 国际收支 and redistribution of income 收入再分配. A possible conflict 冲突 between objectives needs a causal explanation and conditions, not an automatic rule. These broader objectives and policy trade-offs are not required by AS syllabus section 5.1; the core work here is explaining the three policy types and their effects.

fiscal policy 财政政策 uses government taxation and spending. The budget 预算 records planned revenue and expenditure over a period. A budget deficit 预算赤字 means expenditure exceeds revenue; a budget surplus 预算盈余 means revenue exceeds expenditure. national debt 国债 is the outstanding stock of government borrowing, measured at a point in time. The annual deficit is a flow, not the debt itself.

Budget flows and the debt stock

Worked example. Illustrative annual spending is £120bn and revenue £100bn. Debt starts at £500bn. Assume the entire deficit is financed by net borrowing, with no other adjustments.

Next year spending is £118bn and revenue £110bn. Under the same financing assumptions:

The deficit falls from £20bn to £8bn, but debt still rises. Actual debt also depends on financing choices and other adjustments. Debt servicing uses budget resources; whether borrowing is sustainable depends on the cost and capacity to service it, not just the debt's cash size.

Types and reasons for spending

capital spending 资本支出 buys or improves long-lived assets, such as new classroom computers or a new road. current spending 经常性支出 supports ongoing activity, such as teachers' wages and rent for services this year.

A grant to a student is a transfer of income rather than a direct purchase of current output. Government purchases enter $G$; a transfer itself does not, although recipients may later spend it on consumption.

Spending can provide public services and infrastructure, improve access to education or support incomes. It may address market failure and support demand in a downturn. Judge benefits against resource costs and opportunity costs; not every public project automatically raises productivity.

Tax bases and income patterns

A direct tax 直接税 is levied on income or profit, such as income tax or corporation tax. An indirect tax 间接税 is levied on expenditure and collected through suppliers, such as a sales tax. These labels describe the tax base and collection route, not who ultimately bears the burden.

Taxes can raise revenue for services, influence demand, redistribute income or discourage activities with external costs. A tax on a harmful product may discourage consumption, but its effectiveness depends on demand response and alternatives.

A different classification asks how total tax as a share of income changes when income rises:

• progressive tax 累进税: the average rate rises.
• proportional tax 比例税: the average rate stays constant.
• regressive tax 累退税: the average rate falls.

Worked tax bands: total, average and marginal tax

The following annual tax system is hypothetical, not any country's current rules. There are no deductions. Only income within each band faces that band's rate.

Annual income band	Rate
First £10000	0%
Next £10000	10%
Above £20000	20%

At income £25000, the first £10000 faces 0%, the next £10000 faces 10%, and only the remaining £5000 faces 20%.

The average tax rate 平均税率 (ATR) is the share of total income paid in tax:

At income £26000, total tax is £2200. The marginal tax rate 边际税率 (MTR) measures the tax on extra income. This £1000 increment stays inside the 20% band:

A marginal rate does not apply to all earlier income. If an income increment crosses bands, its extra tax must be calculated across the relevant bands.

Practice with solution. At income £15000, total tax is £500. Compare its ATR with the 8% rate at £25000:

The rising average rate makes this system progressive. A flat 10% income tax is proportional. A fixed £1000 charge per person is regressive: its income share falls from $1000/15000\times100\%\approx6.67\%$ to $1000/25000\times100\%=4\%$. Paying more pounds does not itself show a progressive tax. An indirect tax may be regressive relative to income when lower-income households spend a larger share on taxed goods; check their spending patterns and exemptions.

Fiscal policy through AD and AS

expansionary 扩张性 fiscal policy aims to raise demand through higher government purchases or lower taxes. contractionary 紧缩性 fiscal policy uses the reverse changes. A deficit alone does not prove a new discretionary expansion: weak activity can reduce revenue without a deliberate policy change.

Worked explanation. Compare an additional £10bn of government purchases of domestic services with a £10bn household income-tax cut. Purchases directly raise $G$ in $AD=C+I+G+(X-M)$. The tax cut raises disposable income, but households may save part or buy imports, so extra domestic consumption need not equal £10bn. Identify the route to domestic planned spending instead of assuming equal effects.

With substantial spare capacity, extra demand can raise production with limited price pressure. Near capacity, demand mainly raises prices. Employment effects depend on how firms meet extra output, including changes in hours and productivity. Fiscal contraction reverses the demand pressure in this model, but it can reduce output and jobs as well as price pressure. Spending on useful infrastructure or education can have supply effects later.

Evaluate the constraint

Purchases can support demand directly, but planning and delivery may delay them. Income-tax cuts may support consumption, but cautious households may save the gain; imports can absorb part. The timing depends on the measure, not a rule that budgets can change only once a year.

Borrowing may add debt-service costs. crowding out 挤出效应 can occur if government borrowing raises financing costs or public demand draws resources away from private activity. Its extent depends on credit conditions, spare resources and the policy response. It is not automatically large or absent. Match the judgement to the problem, financing and expected response rather than treating one policy as always best.

monetary policy 货币政策 influences monetary and credit conditions through the central bank 中央银行. The required tools are the interest rate 利率, money supply 货币供给 and credit regulation 信贷管制 (restrictions on lending).

Lower policy rates, an increased money supply in the simplified model, or easier lending restrictions can support spending. These are expansionary changes; contractionary changes work in the opposite direction. Banks must be willing to lend and customers willing to borrow for credit to support spending. In the simplified money-market model, an increase in money supply lowers interest rates with money demand unchanged; do not treat this as an unconditional prediction for every policy framework.

Worked borrowing and credit examples

An illustrative £10000 variable-rate loan has no repayments or fees during the year. Its annual rate rises from 6% to 7%.

The extra £100 annual cost may make some projects less attractive. An existing fixed-rate loan need not change immediately. Policy-rate changes influence commercial rates, but pass-through depends on contracts and banks.

A hypothetical borrowing limit falls from four times annual income to three times. With income £40000 per year, the maximum loan falls from £160000 to £120000. The income multiple represents a number of years of that annual income:

This tighter credit rule can limit spending even if the interest rate is unchanged. A borrower who did not need the extra credit may not change spending. Easing a limit does not force borrowing. These are teaching figures, not an actual country's lending rules.

The transmission mechanism

The transmission mechanism 传导机制 links policy to spending, then to output and prices. A higher interest rate can make borrowing dearer and saving more rewarding. Households may reduce credit-financed consumption and firms may reduce investment, but contracts, confidence and income effects affect the response.

If domestic rates rise relative to foreign rates, higher expected returns may attract funds and increase demand for the currency. A stronger exchange rate 汇率 can make exports dearer to foreign buyers and imports cheaper domestically, weakening net exports and AD. But expectations, risk and changes abroad also affect capital flows. Trade quantities depend on contracts and responsiveness to prices. An interest-rate rise alone does not prove that exports must fall.

Effective monetary expansion may raise $C$ or $I$ and shift AD right, like the fiscal AD increase illustrated earlier. With upward-sloping SRAS, real output and price level rise, other things unchanged. Near capacity, price pressure dominates; with spare capacity, output can respond more. Employment depends on how firms meet extra output, including hours and productivity. Rate decisions can change quickly, but borrowing, spending and production take time; there is no fixed lag for every economy or instrument.

Worked comparison. Factories have idle capacity, confidence is weak and policy rates are near zero. Extra public purchases can support demand directly, but delivery, funding and imports affect their impact. Weak confidence may limit borrowing even when credit is cheap; near-zero rates restrict further conventional rate cuts. This does not prove every monetary tool is exhausted or that fiscal action must succeed. Compare feasible measures, their likely response and timing. A different starting interest rate alone does not determine the best policy.

supply-side policy 供给侧政策 aims to raise productivity 生产率 and productive capacity 生产能力, shifting LRAS right. Productivity is useful output per unit of input; raising it need not mean employing more people.

Worked productivity example

In an illustrative factory, 100 workers each work 8 hours per day. Useful training and new machinery raise output per worker-hour from 10 to 12 units, with workers and hours unchanged.

Possible output rises by 20%, with no extra workers in this case. Actual output and sales also require demand, inputs and use of the improved process.

Direct provision: skills, infrastructure and technology

training 培训 can improve relevant skills and reduce a mismatch between jobseekers and vacancies. Suppose an industry closes while local technician vacancies remain. Of 140 trainees, 120 qualify and 90 find suitable jobs. Qualifications alone do not create vacancies: location, demand and access matter. Check completion, matching and whether jobs are additional rather than displaced from other applicants. Retraining alone need not restore sales during a general demand slump.

Better infrastructure 基础设施 can reduce transport delays, improve access to suppliers and lower unit costs. Check reliability, use and the value of saved resources against construction costs. Research support can improve production methods, but success and adoption are uncertain. Funding alone does not prove higher productivity. Construction spending can raise AD before supply benefits arrive; time and quality vary by measure.

Market-based approaches

Lower income tax can increase the reward for extra work or investment, changing the incentive 激励. The response depends on available jobs, finance and people's preferences; some may value more leisure. Lower benefits may change job-search incentives but do not create suitable vacancies by themselves. Consider access and hardship as well as incentives.

privatisation 私有化 transfers state enterprises to private ownership. It may strengthen incentives to control costs, but ownership alone does not create competition or better management. deregulation 放松管制 can remove unnecessary entry barriers and encourage competition. Removing useful safety or environmental rules can also impose costs.

Comparing tools and evaluating claims

Worked comparison. Region A has workers with unsuitable skills and poor transport. Region B has qualified workers and efficient transport but few orders. Direct training and infrastructure may address A's gaps if useful and used. Tax incentives and easier entry can encourage work and investment, but do not automatically supply missing skills or transport. B's immediate constraint is weak demand, so extra capacity may remain unused. Compare cause, response, cost and timing; combinations may complement each other.

A useful supply improvement can support higher output with less price pressure in the stated model. It is not a promise of no costs. Funding uses resources, taxes can change spending and incentives, and gains may arrive unevenly or late. Judge quality, opportunity costs, distribution, demand and timing. Distinguish a lower price level in a diagram from a sustained change in the inflation rate.

Diagnose rising prices. Economy A has strong spending near capacity. B has a fuel-cost shock and weak sales. Monetary tightening may restrain demand pressure in A. In B, weaker demand does not directly remove the fuel shortage and may reduce output further. Supply measures may reduce costs but may take time or fail to address the specific shock. Compare source, persistence and feasible response rather than choosing an instrument from inflation alone.

Improve the assessment. "Training reduces unemployment, but there is a time lag" is incomplete. Explain that training can reduce a skills mismatch if vacancies need the new skills. Completion and matching take time, so lost income may persist. If firms lack orders, demand support may address the immediate sales constraint more directly. A judgement compares mechanisms, scale, time and costs; merely naming a lag is not complete evaluation.

Attempt before checking. Spending is £90bn/year and revenue £84bn/year; starting debt is £300bn. Assume the deficit is financed by net borrowing with no other adjustments. Find deficit and closing debt. A worker pays £3000 tax on £30000 income, then £3200 on £31000: find ATR and MTR on the extra income.

Final check. Explain a monetary tool other than interest rates. Explain why training can raise capacity without guaranteeing jobs. Why can a government package that raises income tax and funds apprenticeships be both fiscal and supply-side?

Check after attempting. Easing credit regulation may allow more financed spending if banks and borrowers respond; increased money supply may lower rates in the simplified model with unchanged money demand. Training can improve productivity or matching, but suitable vacancies and demand remain necessary. Tax and purchases are fiscal tools, while useful apprenticeships can also improve skills and capacity. Explain demand and supply effects separately and state their time periods.

Vocabulary card meanings

redistribution of income: Shifting income from some households to others, often by tax. demand-side policies: Government actions aimed at the level of total spending. privatisation: Selling firms owned by the state to private owners. deregulation: Removing rules that limit how firms may compete. capital spending: Public money spent on long-lived assets such as roads. current spending: Public money used up within the year, such as wages. average tax rate: Tax paid divided by total income. marginal tax rate: The share taken in tax from one extra unit of income. credit regulation: Rules that limit how much households and firms may borrow.

international trade 国际贸易 lets residents exchange goods and services across borders. To specialise 专业化 means to concentrate on producing particular goods or services. Specialisation and trade can increase consumption choices; calculate the possible gain and then consider costs and adjustment.

Absolute and comparative advantage

An absolute advantage 绝对优势 means producing more with the same stated resources. A comparative advantage 比较优势 means producing at lower opportunity cost 机会成本. They answer different questions.

Illustrative maximum outputs with the same quantity of resources in each country; assume straight-line PPCs and unchanged technology:

Country	Wine only	Cloth only
A	20 units	10 units
B	5 units	5 units

A has an absolute advantage in both. To find comparative advantage, divide output forgone by output gained:

B gives up less wine per cloth, so has comparative advantage in cloth. A gives up less cloth per wine, so has comparative advantage in wine. One country being more productive in both goods does not remove possible gains from trade.

Gains in consumption and the trading possibility curve

Worked exchange ratio. B gives up 1 wine to make 1 cloth; A gives up 2 wine to make it. At 1 cloth for 1.5 wine, B receives 0.5 wine more than its cost and A saves 0.5 wine compared with making cloth at home. Both strictly gain per traded unit when the wine-per-cloth ratio is between 1 and 2, before trading costs. At an endpoint, one side is indifferent in this model.

Worked consumption bundles. Without trade, A makes 16 wine and 2 cloth; B makes 3 wine and 2 cloth. Both lie on their own PPCs. After specialisation, A makes 20 wine and B makes 5 cloth. B sends 2.5 cloth to A in exchange for 3.75 wine, consistent with 1.5 wine per cloth.

Country	Wine consumed	Cloth consumed
A	20 − 3.75 = 16.25	0 + 2.5 = 2.5
B	0 + 3.75 = 3.75	5 − 2.5 = 2.5

Both can consume more of both goods than in the stated no-trade bundles. Total consumption equals total production: 20 wine and 5 cloth. The example shows a possible gain, not a promise that all trades or all households gain equally.

A trading possibility curve 贸易可能性曲线 shows consumption combinations available through production and trade at a stated exchange ratio. It can extend beyond a country's production PPC without moving that PPC. The country's technology and resources have not changed; trade changes access to goods. Here B can supply at most 5 cloth, so the shown trading segment is limited to those imports.

Terms of trade: prices, not the trade balance

terms of trade 贸易条件 compare export prices with import prices, using indices with the same base period:

With export-price index 110 and import-price index 105:

A fixed export quantity now buys about 4.8% more imports than in the base period. The index does not measure export volumes or the current-account balance. An improvement could reflect higher export prices or lower import prices, with different effects on sales and costs. Commodity demand, supply and production costs influence the ratio. Dearer exports may sell fewer units; improved terms of trade do not automatically mean higher export revenue or welfare.

Transfer task with solution. Terms of trade fall from 95 to 85 while the import-price index stays 120. Find both export-price indices and their change:

Export prices fall 12 index points, not 10. Distinguish an index-point change from a percentage change.

Limits and distribution of gains

If the calculated saving is 0.5 wine per cloth but shipping and border costs use resources worth 0.6 wine per cloth, the saving is lost. The simple model holds technology and resources fixed, assumes constant opportunity costs, and omits trading costs and barriers. Moving workers and capital into expanding industries can require time, retraining and investment. Changing costs, product quality and reliance on one export also matter. Trade can widen choice and competition; assess adjustment losses and the distribution of gains instead of assuming everyone benefits equally.

protectionism 贸易保护主义 shields home producers from foreign competition. Explain each tool's direct mechanism before judging its effects:

• A tariff 关税 taxes imports. In the small-country model below, it raises the domestic price while imports remain available.
• A quota 配额 caps the quantity imported. A quota above existing imports may not bind.
• An export subsidy 补贴 pays exporters, reducing effective costs and potentially expanding foreign sales. It requires public funding; it does not automatically raise every domestic price.
• An embargo 禁运 bans specified trade, potentially increasing the cost of alternatives.
• administrative barriers 行政壁垒 are excessive procedures that hinder imports. Distinguish these from useful safety or environmental standards.

Worked tariff: imports and revenue

Assume a small country facing an unchanged world price, no evasion and full pass-through of the tariff into the domestic price. Prices are £/unit; quantities are million units/year. At world price 4, domestic supply is 2 and demand 8. A tariff of 2 raises the domestic price to 6: supply rises to 4 and demand falls to 6.

Use imports after the tariff. Multiplying the tariff by all domestic consumption or the old import quantity gives the wrong revenue.

Effects on stakeholders

A stakeholder 利益相关者 is a person or group affected. Consumers pay 6 rather than 4 and buy 6 rather than 8 million units, reducing purchasing power. Home producers sell 4 rather than 2 million units at the higher price, but higher-cost production may expand. Government receives £4 million/year while taxable imports continue; revenue is a transfer, not evidence of a net national gain. Foreign suppliers sell fewer units in this model. Domestic firms using imported inputs can face higher costs. Compare the gains and losses and wider responses.

A quota need not give gains to the same recipient

Suppose an import quota of 2 million units/year produces the same domestic price 6 while foreign supply remains available at 4. A licence holder buying at 4 and selling at 6 may receive the difference. Government can capture some or all of it by auctioning licences; foreign suppliers may receive gains if they hold licences or market power. Do not claim that every quota generates no government receipts or always benefits domestic importers. A tariff fixes a tax per imported unit; a quota fixes the permitted quantity, so their response to changing demand can differ.

Evaluate arguments for and against protection

An infant industry 幼稚产业 may need time to learn or reach efficient scale. A credible temporary-protection case needs evidence of falling costs, measurable targets and an end date. Protection may instead preserve inefficiency and impose costs on consumers. Strategic capacity and time for workers to adjust may also matter.

dumping 倾销 concerns export prices below normal value, commonly compared with home-market prices or assessed using costs. Cheap imports alone do not prove an intention to destroy competitors. WTO definition.

retaliation 报复 by trading partners can weaken export sales. Judge a specified tool against its objectives, duration, consumer and input costs, possible foreign responses and alternatives. A justified conclusion weighs the actual conditions; it does not simply repeat that protection saves jobs or raises prices.

The balance of payments 国际收支 records transactions between residents and non-residents during a period. The current account 经常账户 is one part; it records goods, services, primary income and secondary income. A receipt is a credit, and a payment is a debit. Use the same period, currency and sign convention throughout a calculation.

Classify transactions before calculating

• trade in goods 货物贸易: exports and imports of physical products. A resident firm exporting machinery receives a goods credit.
• trade in services 服务贸易: services supplied across the resident/non-resident boundary. A foreign visitor paying a resident hotel creates a service credit.
• primary income 初次收入: income from labour or ownership of assets across borders. Interest received by a resident on a foreign bond is a credit; interest paid abroad is a debit.
• secondary income 二次收入: current transfers without an equivalent item received in return, such as a household gift from abroad.

Buying an existing foreign bond is a financial-asset transaction, not primary income. Its later interest is income. Goods made and consumed domestically are not exports simply because they are valuable.

Worked account: trade surplus but current-account deficit

Illustrative annual flows, all £bn/year:

Component	Receipts	Payments
Goods	80	100
Services	50	20
Primary income	5	20
Secondary income	2	5

For goods and services, receipts are export earnings and payments are import expenditure. Let $B_g$ and $B_s$ be their balances, $B_t$ the combined trade balance, and $N_p$ and $N_s$ net primary and secondary income. For income, $R$ means receipts and $P$ payments.

Goods are in deficit, services in surplus, and combined trade in surplus. Income and transfers still matter:

A current account deficit 经常账户赤字 means total current-account debits exceed credits. A current account surplus 经常账户盈余 means credits exceed debits. Neither is defined solely by imports and exports. This example has a £10bn trade surplus alongside an £8bn current-account deficit.

Explain the cause before choosing a policy

Transfer task with solution. Starting from CAB $=-8$, compare an extra £12bn/year of service exports with an extra £12bn/year of interest payments abroad. Hold all other flows constant.

Stronger overseas demand can raise exports; stronger domestic spending can raise imports. Exchange rates, relative prices, product quality and capacity influence trade. Income payments and transfers can change CAB even when trade is unchanged. A single balance does not identify its cause.

Assess consequences and financing

Compare two economies with the same deficit. Economy A imports machinery for a viable new export industry. Economy B persistently loses overseas customers while paying more income abroad. A's imports may improve future capacity and earnings if the investment succeeds. B's lost sales may weaken demand and employment, although other expenditure can offset this. Size alone cannot settle the judgement.

A deficit requires counterpart financing or asset/reserve changes in the wider accounts. Borrowing, equity investment and selling assets differ: external debt does not automatically rise by the current-account deficit. Consider persistence, financing terms, future income payments, exchange-rate pressures and productive use of imports. A surplus can reflect strong competitiveness or weak domestic spending; it is not automatically a welfare gain.

An exchange rate 汇率 is the price of one currency in another. State the quote before describing a rise or fall. For example, US$1.40 per pound means £1 buys US$1.40. If this rises to US$1.60 per pound, the pound buys more dollars: appreciation 升值. A fall in the dollar-per-pound quote is depreciation 贬值. A quote in pounds per dollar moves the opposite way.

The floating currency market

A floating exchange rate 浮动汇率 adjusts where demand for the currency meets supply. In the market for pounds, a buyer exchanging dollars for pounds to pay a UK exporter demands pounds. A UK importer exchanging pounds for dollars supplies pounds. Follow the currency exchanged, not the buyer's nationality.

With currency supply unchanged, stronger demand for pounds raises foreign currency per pound: appreciation. Higher export demand or inward investment may raise demand. With currency demand unchanged, UK residents buying more foreign goods can increase supply of pounds and cause depreciation. Outward investment can also increase supply. Higher relative interest returns may attract funds, but expectations and risk can offset that response.

A change in the pound's own price causes movement along a curve; a change in export demand or investment flows can shift a curve. State other influences held constant.

For context, a fixed exchange rate 固定汇率 is maintained at a stated rate or band using intervention or other policy measures. Maintaining it can require reserves and policy adjustments. A managed exchange rate 管理浮动汇率 combines market forces with intervention to influence the rate. These arrangements describe currency pricing; a current-account surplus describes a different account and does not identify the currency system. The required AS model here focuses on floating rates.

Worked export conversion

A UK machine keeps its £200 price. The rate rises from US$1.40/£ to US$1.60/£. Assume the change is passed through to the US buyer. Let $P_{\mathrm{US}}$ be its dollar price, $P_{\mathrm{UK}}$ its pound price and $e$ dollars per pound. Multiply so pounds cancel:

The machine becomes dearer in dollars. Sales may fall, depending on demand, contracts, quality and how firms adjust prices. Conversion arithmetic alone does not establish the quantity response.

Worked import conversion

A UK firm owes a fixed US$700 invoice. Use the same rates. Dividing dollars by dollars per pound gives pounds:

Appreciation lowers this invoice's pound cost. Always check the units rather than memorising that imports mean multiplication or division.

Price effects do not settle the trade balance

Transfer task with solution. The pound depreciates from US$1.40/£ to US$1.20/£. A fixed £200 export now costs US$240, while a fixed US$700 import costs about £583.33:

Export receipts and import spending equal price times quantity. Buyers may be tied to contracts, essential imports may be hard to replace, and exporters may lack capacity. Cheaper exports do not automatically mean higher receipts; dearer imports do not automatically mean a lower import bill. Assess demand responsiveness, timing and the currency in which values are measured. The whole current account also includes income and transfers. A short technical label alone cannot replace this reasoning.

AD/AS: demand and imported costs

A depreciation can raise net exports if quantities respond, increasing aggregate demand 总需求 (AD). With upward-sloping short-run supply and other influences unchanged, AD shifting right raises real output and the price level. Employment also depends on how firms meet extra demand, including hours and productivity.

Dearer imported fuel, components or machinery can increase production costs. Higher costs shift short-run aggregate supply 短期总供给 (SRAS) left: holding AD unchanged, real output falls and the price level rises. If demand and cost effects occur together, their relative size determines the output change. Depreciation does not automatically remove structural unemployment or guarantee higher output and jobs.

First identify the component causing the imbalance, its persistence and financing. The aim of stability does not mean forcing a zero balance every year at any cost. Compare outcomes for growth, jobs, inflation and future income payments.

Fiscal and monetary policies: reducing expenditure

expenditure-reducing 支出减少 policies lower domestic demand. Higher income taxes or lower government purchases may reduce spending and imports. If demand is already weak, output and employment can also fall. The import response depends on which spending changes and the availability of domestic alternatives.

Higher interest rates may reduce borrowing and spending, including imports. They may also attract capital and appreciate the currency, making exports dearer abroad and imports cheaper at home. Expectations and risk matter. Explain both channels before claiming that tighter monetary policy must improve CAB.

Protection and relative prices: switching expenditure

expenditure-switching 支出转换 changes relative prices to favour domestic goods. A tariff may reduce imports; an export subsidy may support foreign sales but directly costs public funds. A tariff's receipts and a subsidy's costs have different effects on the government budget. A quota changes permitted quantities and its revenue effects depend on licence arrangements.

Depreciation can also change relative prices, but a floating rate is not simply set lower by government command. Monetary policy can influence it with uncertain responses. Higher import costs, retaliation, weak demand responses and limited capacity can offset gains. Compare a specified policy with feasible alternatives rather than promising all effects from its label.

Supply-side policies: address the actual constraint

Supply-side policy can improve competitiveness 竞争力 through productivity, quality, costs and reliability. Compare two exporters: A cannot deliver reliably because a port is congested; B has spare capacity but foreign buyers no longer want its product. Port investment may address A's constraint; B may need product development or a different market. The same training programme need not solve both.

Skills, infrastructure and technology use resources and may take time. Higher capacity does not guarantee sales. Judge opportunity cost, the actual bottleneck and overseas demand before predicting higher exports.

Diagnose before giving a conclusion

Worked judgement. CAB is −£8bn/year despite a £10bn trade surplus because net primary and secondary income together are −£18bn/year. A tariff targets imports, not the income payments causing the imbalance. It could still change trade, but consumer and input costs, the size of the response and retaliation matter. Recommend a policy using the identified cause and compare benefits with other objectives.

Attempt before checking. Annual balances, in £bn/year: goods−12, services+8, primary income−7, secondary income+1. Find CAB. Then £1 falls from US$1.50 to US$1.20. Find the dollar price of a fixed £150 export. Does the lower foreign price prove that CAB will improve?

Solution. Add all four components:

Previously the export cost US$225. The pound depreciates; export quantities, import costs, income and transfers still determine the overall account.

Improve a judgement. Replace “depreciation will solve the deficit and unemployment, so it is the best policy” with a conditional explanation. A lower foreign price can support export demand if buyers respond and firms can expand. Dearer inputs may increase costs and prices, while essential imports can keep the import bill high. Judge the cause of the imbalance, demand responsiveness, capacity and timing, then compare alternatives that address the problem. A justified conclusion follows those conditions instead of promising that one tool meets every objective.

Vocabulary card meanings

international trade: Buying and selling across national borders. embargo: A government ban on trade with a named country or product. administrative barriers: Paperwork and rules that make imports harder without using a tariff. infant industry: A new home industry not yet able to match foreign rivals. current account deficit: Paying more to other countries for trade and income than is received. current account surplus: Receiving more from other countries for trade and income than is paid. managed exchange rate: A currency price that the central bank steers rather than leaving fully free. trading possibility curve: The combinations of goods a country can enjoy once it specialises and trades.

utility 效用 means satisfaction. The numbers below are illustrative utility units for one person. They are a model, not objective measurements that can be compared across people.

Calculate total and marginal utility

total utility 总效用 is satisfaction from the whole quantity consumed. marginal utility 边际效用 is the change in satisfaction from an extra unit. With other influences unchanged, diminishing marginal utility 边际效用递减 means each extra unit gives less additional satisfaction.

Quantity	TU	MU of extra unit
0	0	—
1	12	12
2	22	10
3	30	8
4	36	6
5	40	4
6	40	0
7	38	−2

Worked example. MU is a change, not TU divided by quantity. For one-unit steps:

TU rises while MU is positive, stays unchanged for the zero-MU sixth unit, and falls for the negative-MU seventh unit. Diminishing MU does not itself imply that TU always rises. Here TU reaches 40 at both 5 and 6 units.

Compare all feasible bundles

Income is £8; A costs £2/unit and B £1/unit. Only whole units can be bought, with at most 4A and 8B available. Assume the utilities of A and B add without interaction. A's MU for units 1–4 is 12, 8, 4, 0. B's MU for units 1–8 is 7, 6, 5, 4, 3, 2, 1, 0. Add successive MU values to calculate each good's TU.

For example, 2A gives $12+8=20$ utility and 4B gives $7+6+5+4=22$. The budget and combined-utility relationships are:

Since all additional utility in the available ranges is non-negative, a best bundle can be found among the whole-unit bundles that spend the full budget:

A units	B units	Spending	Combined TU
0	8	£8	28
1	6	£8	39
2	4	£8	42
3	2	£8	37
4	0	£8	24

The best listed bundle is 2A and 4B. A high first-unit MU alone does not identify the best complete budget allocation.

The equi-marginal principle and its conditions

The equi-marginal principle 等边际原则 compares extra utility per pound spent. For a usual interior optimum with divisible goods and a binding budget, the ratios are equal:

At the chosen 2A/4B bundle, the last A unit gives 8 utility and the last B unit gives 4:

Compare MU per pound, not MU alone. If a small transfer of spending can buy more extra utility from A than it gives up from B, reallocating can improve satisfaction. At a corner or with indivisible goods, exact equality may not be possible. Check feasible bundles and the budget instead of forcing the equation to hold.

A change in tastes: recheck the optimum

Keep prices and income unchanged but double every MU value for B, leaving A unchanged. Combined utility at each full-budget bundle becomes:

A units	B units	New TU
0	8	56
1	6	66
2	4	64
3	2	50
4	0	24

The best listed bundle becomes 1A/6B: more B and less A, with the same £8 budget. Quantity can change because preferences change. A price fall is not the only reason someone might buy more.

Derive a demand response with other influences fixed

Return to the original preferences, keep income £8 and B's price £1, then cut A's price from £2 to £1. Within the same available quantities, the new full-budget comparisons are:

A units	B units	TU with original tastes
0	8	28
1	7	40
2	6	47
3	5	49
4	4	46

A's chosen quantity rises from 2 to 3 as its own price falls from £2 to £1: two points on this person's demand curve 需求曲线. The optimum is found by comparison even though exact MU/P equality is unavailable for this whole-unit choice. Holding preferences, income and the other price fixed is essential when deriving an own-price response.

Limitations of marginal-utility reasoning

The result is conditional on the person's preferences, budget and available goods. The tables also assume utility from A and B can be added without interaction. Utility numbers are illustrative rather than objective readings of satisfaction. Real choices may involve habits, incomplete information and changing preferences, and people need not explicitly calculate a table. The model gives a conditional prediction, not a universal instruction to buy 2A and 4B.

Calculate the affordable set

A budget line 预算线 shows bundles that use all income at the stated prices. Points below it are affordable; points above it are not. For divisible goods X and Y, spending cannot exceed income:

On the line, equality holds. With X horizontal, rearrange to express Y:

The vertical intercept is $I/P_Y$, the horizontal intercept is $I/P_X$, and the slope is $-P_X/P_Y$.

Worked example. Income is £24; X costs £4/unit and Y costs £1/unit. Find the intercepts and check the bundle (3X,12Y).

The bundle uses the full budget. This establishes feasibility, not maximum satisfaction.

Income shifts and price pivots

Start from the same income and prices. Each change below occurs on its own; quantities are in units.

Change	X intercept	Y intercept
Income rises to £36	9	36
X price falls to £2/unit	12	24
Y price rises to £2/unit	6	12

Higher money income moves both intercepts out in the same proportion, leaving the slope unchanged. Lower income shifts the line inward. A change in only X's price pivots the line about the unchanged Y intercept; a lower X price makes it flatter. A change in only Y's price keeps the X intercept fixed.

A parallel outward shift need not prove that money income rose. At unchanged £24 income, reducing both prices to two-thirds of their old levels also moves the intercepts from (6X,24Y) to (9X,36Y). The price ratio is unchanged. A taste change affects the preference map, not the budget line if income and prices stay fixed.

Rank satisfaction, then find the best affordable bundle

An indifference curve 无差异曲线 joins bundles giving the same satisfaction. An indifference map contains several such curves. In the usual model both goods are desirable: more of either, with the other unchanged, is preferred. Curves therefore slope downward; giving up one good requires more of the other to keep satisfaction unchanged. The usual bowed-in shape represents a declining willingness to give up Y for another X as X becomes more plentiful. Curves do not cross when preferences are consistent; a crossing would give contradictory rankings. These are model assumptions, not shapes required for every possible preference pattern.

Unlike the illustrative utility tables, this model needs only rankings of satisfaction, not an objective numerical measurement of it.

Worked example. In the illustrated map, (2X,18Y), (3X,12Y) and (6X,6Y) lie on the same curve. With prices £4/unit and £1/unit:

Equal satisfaction does not imply equal spending. Only (3,12) is affordable among these three bundles with £24 income.

Consumer equilibrium 消费者均衡 is the most preferred affordable bundle. In this smooth example, the budget line is tangent to the highest reachable curve at A (3,12). A lower curve gives less satisfaction; a higher one has no affordable point.

Corner counterexample. Suppose preferences are represented by $U=3Q_X+Q_Y$, with £24 income and the same prices. The extra satisfaction per pound is:

Y gives more satisfaction per pound at every bundle. The best choice is 0X and 24Y, a corner. Its straight indifference curve is not tangent to the budget line. Divisible goods, smooth convex curves and an interior choice are extra conditions behind the familiar tangent diagram.

Separate income and substitution effects

A price fall changes relative prices and purchasing power. The substitution effect 替代效应 isolates the response to the changed price ratio while keeping satisfaction unchanged. The income effect 收入效应 then isolates the response to the gain in real purchasing power at the new price ratio. The total price effect is their sum. Money income need not change for real purchasing power to change.

In the following smooth illustrative map, income stays £24 and Y costs £1/unit. X's price falls from £4 to £1/unit. The old optimum is A (3X,12Y); the new optimum is B (12X,12Y).

Draw a comparison line parallel to the new budget line, touching the original indifference curve at C (6X,6Y). This compensated line has the new price ratio but reduces income enough to keep satisfaction unchanged. C is a comparison point, not the final choice with actual income.

Worked example. Use the X coordinates to calculate each effect:

This is a normal good 正常品 example: substitution and income effects both raise X. On the new-price comparison line C costs £12; restoring actual £24 income leads to B on a higher curve. The diagram is consistent with the stated coordinates and budgets; it is an illustration, not observed consumer data.

Inferior goods and the Giffen exception

For an inferior good 低档品, higher real income reduces the quantity chosen. The income effect opposes substitution towards the now cheaper good. If substitution is stronger, quantity still rises when price falls. A Giffen good 吉芬品 is the special inferior case where the opposing income effect is stronger, so quantity falls when its price falls, with other influences fixed. Inferiority alone does not establish Giffen behaviour.

These separate hypothetical price-fall cases are not extra points on the preceding map. All entries are changes in X quantity, in units.

Case	Substitution	Income	Total
Normal	+3	+6	+9
Inferior, not Giffen	+3	−1	+2
Giffen	+3	−5	−2

The total follows from adding the two effects. Classify the inferior cases by the negative income effect; distinguish Giffen by its size relative to substitution. An upward-sloping own-price demand response requires the Giffen condition, not simply a cheap product or a low-income buyer.

Evaluate the model with the diagram

For an exam response about a price change, label both goods, show the budget pivot and the old/new highest reachable curves, then add the compensated line to distinguish the two effects. Compare normal, inferior and Giffen responses before judging the claim that a lower price always raises quantity. A pivot alone cannot prove that claim.

The model assumes consistent preferences, the ability to rank bundles and a choice of the most preferred affordable bundle. Two goods simplify the many alternatives available in real markets. Habits and incomplete information can affect choice; advertising or changing tastes can change the map itself. Indivisible goods or corner choices can prevent a smooth interior tangency. These limitations explain when a prediction may fail; simply listing them does not evaluate a particular price response.

Check yourself. Intercepts rise from (6X,24Y) to (9X,36Y). Give an income explanation and a price explanation. Then explain why a +3-unit substitution effect and a −5-unit income effect make a price fall reduce total quantity by 2 units.

Answer. At unchanged prices, income £24→£36 produces the shift; at unchanged £24 income, both prices falling to two-thirds of their old levels also does so. In the effects case, the good is inferior and the opposing income effect is larger, so it is Giffen over the illustrated change. The total price effect is $+3+(-5)=-2$ units.

Efficiency has several meanings. A plant can cut its cost per unit without making the right quantity for society, and a useful innovation can still leave someone worse off. State which test is being applied.

Productive efficiency: test cost per unit

Productive efficiency 生产效率 means producing at the lowest possible average cost 平均成本. Resources are used without avoidable waste. For a firm, compare its output with the minimum of the relevant average-cost curve; a large output alone does not establish efficiency.

Worked example. A plant has these illustrative monthly costs at unchanged input prices and product quality. Calculate average cost, then identify the lowest observed value.

Output (units/month)	Total cost (£/month)	AC (£/unit)
50	450	9
100	800	8
150	1350	9

100 units/month has the lowest AC among the observations. The full cost curve is needed to establish the overall minimum. Raising output from 100 to 150 lowers neither total cost nor average cost. A fall in input prices or product quality could also lower measured unit cost, so these must be controlled before inferring an improvement in the use of resources.

Allocative efficiency: compare the benefit and cost of extra output

Allocative efficiency 配置效率 means resources are allocated to the output mix that maximises net social benefit. The condition is marginal social benefit 边际社会收益 equal to marginal social cost 边际社会成本, with no remaining beneficial reallocation. If marginal benefit exceeds marginal cost 边际成本, a small expansion adds net benefit; if marginal cost exceeds marginal benefit, reduce output. The usual crossing with falling benefit and rising cost identifies the best quantity.

The familiar condition $P=MC$ applies when price represents consumers' marginal benefit and private benefits/costs match social ones. A price equal to private marginal cost does not prove allocative efficiency if external costs or benefits are ignored.

Worked example. In this separate continuous-output illustration, there are no external costs or benefits. MB means the benefit to consumers from extra output and MC its resource cost. Values are illustrative, not the cost data in the previous example.

Output (units/month)	MB (£/unit)	MC (£/unit)	Direction
3	9	5	Expand
5	7	7	Best margin
7	5	9	Reduce

At output near 3, extra output is worth more than it costs. Near 7, reducing output saves more cost than the benefit lost. At 5, the two margins are equal. This is a test of quantity and benefit, not a test of minimum average cost.

Productive and allocative efficiency need not occur at the same output. For example, the output where a demand curve meets marginal cost need not be at the minimum of average cost. Inspect each condition separately rather than treating low price, normal profit or high output as proof of both.

Pareto optimality: can someone gain without anyone losing?

A Pareto improvement makes at least one person better off and nobody worse off. Pareto optimality 帕累托最优 means no feasible Pareto improvement remains. It does not establish fairness or a unique best distribution.

Worked example. The following four outcomes are the complete feasible list in a toy model. A larger score means a better outcome for that person; the scores only rank outcomes and are not added across people.

Outcome	Person 1	Person 2
A	10	10
B	12	10
C	13	9
D	11	11

A→B helps Person 1 without harming Person 2; A→D helps both. A is therefore not Pareto optimal. B, C and D are each Pareto optimal within this complete feasible list: moving between them helps one person at the expense of the other. Their different distributions show why Pareto optimality alone does not select a fair outcome. If another feasible outcome were added, the judgement would need to be checked again.

Training or removing waste may create the resources for a Pareto improvement, but consider the training cost and who gains or loses. A rise in aggregate benefit is not by itself proof that nobody is worse off.

Dynamic efficiency: improvements over time

Dynamic efficiency 动态效率 concerns improvements through innovation, better methods and better products over time. It changes future production or consumption possibilities. Productive and allocative efficiency test the use and allocation of resources at a particular time.

Worked example. Successful research creates a process that makes the same 100 units/month, at unchanged product quality and input prices, for £600/month rather than £800/month:

This is evidence of an improvement in production possibilities. It does not prove the firm is at its new minimum AC, chooses the allocatively efficient output, passes the saving to consumers, or recovers the research cost. Those require further information. A new product or better quality can also be a dynamic improvement even when its unit cost does not fall.

Competition can encourage innovation by rewarding better quality or lower costs. Market power and profits may provide finance for a costly project, but they do not guarantee that it will be undertaken. Conversely, competition does not automatically prevent research or access to finance. Judge the incentive, available resources, ability to keep the gains from invention and actual results. Do not claim that a competitive firm cannot innovate simply because it earns normal profit in a long-run model.

Market failure: identify the mechanism and the allocation problem

Market failure 市场失灵 occurs when the market fails to allocate resources efficiently. A high price, unequal income or low profit is not sufficient evidence by itself. Identify the missing cost or benefit, the incentive problem or the constraint, and explain its effect on the quantity or use of resources.

Market power. A sole seller may restrict output even though buyers value extra output more than its full resource cost. Such mutually beneficial output is left unproduced. Compare benefit and cost at the margin; a high price alone does not prove this gap.

Externalities 外部性. A factory may ignore damage its smoke causes to nearby households. Its private calculation then understates the cost to society, potentially causing excess output. The next section distinguishes the production and consumption cases, including external benefits.

Public goods 公共物品 are non-rival and non-excludable. In a flood-warning broadcast, another listener does not reduce anyone else's reception and non-payers cannot be excluded from the signal. Residents can benefit while leaving others to pay, weakening voluntary private funding. The sum of benefits may justify provision even when a private seller cannot collect enough payment to cover cost. The allocation problem is possible underprovision, not an absence of value. Public ownership is not the definition; test rivalry and exclusion.

Information failure 信息失灵. If a buyer cannot assess a machine's safety, they may buy a model with hidden defects that they would reject with better information. Expected and informed benefits differ, potentially causing a poor allocation. The cost of acquiring information also matters.

Factor immobility 要素不流动性. Workers or capital may not move quickly to more useful activities despite higher returns. Retraining, relocation and other barriers delay adjustment. However, moving and training use real resources: unemployment or a price difference alone does not prove inefficiency. Explain why the barrier prevents a beneficial reallocation after allowing for those costs.

Check yourself. A process lowers unit resource cost from £8 to £6 at fixed quality and input prices. At the chosen output, marginal social benefit is £5/unit and marginal social cost £7/unit. A resident loses from the associated noise. Which efficiency claims follow?

Answer. The innovation is evidence of a dynamic improvement and lower cost at that output, but does not prove minimum AC. Because marginal social cost exceeds marginal social benefit at the chosen margin, reducing output would raise net social benefit: allocative efficiency has not been achieved. The resident's loss means the change is not a Pareto improvement as described, even if other people gain more in total.

A factory's smoke may harm neighbours who did not choose its output. A vaccination may protect people other than the person receiving it. The private decision and its full effect on society can therefore differ. An externality is a cost or benefit affecting a third party outside the transaction, not simply any price paid by the buyer.

Calculate total private, external and social values

A private cost 私人成本 falls on the decision-maker; an external cost 外部成本 falls on third parties. The social cost 社会成本 includes both. A private benefit 私人收益 accrues to participants; an external benefit 外部收益 accrues to other people. The social benefit 社会收益 includes both.

Worked example: cost. A factory uses resources costing £800/month and causes pollution damage of £200/month. Assume these are all relevant costs for that output:

The factory's private calculation omits the neighbours' damage. A compensation payment is a transfer between parties, not another resource cost to add on top of the same damage. Count each effect once; any separate administrative or prevention cost would need its own treatment.

Worked example: benefit. In a separate training programme, participants gain benefits valued at £1200/year and others gain £400/year as skills spread:

These are valuations of benefits, not just sales receipts. A price paid or received is not automatically the full benefit or resource cost to society. Use the same activity, quantity and time period when adding components.

Marginal values concern an extra unit

Marginal analysis 边际分析 compares changes. Marginal private cost 边际私人成本 (MPC) and marginal private benefit 边际私人收益 (MPB) concern the private cost and benefit of extra output or consumption. Marginal external cost 边际外部成本 (MEC) and marginal external benefit 边际外部收益 (MEB) concern its effects on third parties. MSC and MSB are the marginal social cost and benefit, defined in the efficiency section.

Worked example. An extra unit of production uses £7 of the firm's resources and imposes £2 of external damage:

In a separate service, an extra unit gives its user £5 of benefit and others £2:

These extra-unit values are not totals or averages. Dividing total social cost by quantity gives an average, not necessarily the marginal cost.

Use private and social equilibrium conditions separately

In the simple competitive-market model without corrective intervention, output is where $MPB=MPC$. The social test is $MSB=MSC$. With falling marginal benefit and rising marginal cost, the social intersection maximises net benefit. Monopoly, taxes or other distortions require separate treatment; the private intersection is not a universal rule for every market structure.

The following four diagrams are separate illustrations. Each isolates one spillover and assumes other distortions absent. A negative externality 负外部性 harms third parties; a positive externality 正外部性 benefits them. The diagram's cost/benefit convention must be stated so a spillover is counted once.

Negative production externality

Factory pollution adds damage to production's private cost, so MSC exceeds MPC. With no consumption spillover, MPB=MSB. The competitive market produces too much.

Worked example. Let $MPB=MSB=12-Q$, $MPC=2+Q$ and $MSC=4+Q$. Here $Q$ is the numerical value of output in units/month, and each marginal function gives £/unit. These are illustrative linear relationships, not observed data.

Positive consumption externality

Vaccination can reduce infection risks for other people, so MSB exceeds MPB. With no production spillover, MPC=MSC. The market consumes too little because the private decision omits benefits to others.

Worked example. Let $MPB=10-Q$, $MSB=12-Q$ and $MPC=MSC=2+Q$, in the same output and marginal-value units:

Positive production externality

Research by one firm can help other producers. In the usual production diagram, this spillover benefit is deducted from net marginal social cost, putting net MSC below MPC. Do not also add the same benefit to MSB; that would count it twice. This representation concerns a net cost after the external benefit, not a claim that society physically uses fewer inputs than the firm.

In this separate example, $MPB=MSB=12-Q$, $MPC=4+Q$ and net $MSC=2+Q$. Applying the private and social conditions gives $Q_m=4$ and $Q^*=5$ units/month.

Negative consumption externality

Loud leisure activity can disturb neighbours. The usual consumption diagram deducts that harm from net marginal social benefit, putting net MSB below MPB. With no production spillover, MPC=MSC. Do not also add the same harm to MSC.

In this separate example, $MPB=12-Q$, net $MSB=10-Q$ and $MPC=MSC=2+Q$. The private and social conditions give $Q_m=5$ and $Q^*=4$ units/month.

Calculate the loss on excess or missing units

Deadweight welfare loss 无谓损失 is net social benefit lost because the quantity differs from the optimum. With excess output, social cost exceeds social benefit on the excess units. With underprovision, social benefit exceeds social cost on missing units. It is not necessarily the entire external cost or benefit.

Worked example. In each of the four linear diagrams, market output differs from the optimum by 1 unit/month. At market output, the social marginal gap is £2/unit and closes linearly to zero at the optimum. The shaded area is therefore a triangle:

A vertical gap has units £/unit; multiplying by the horizontal quantity range gives £/month. The one-half factor applies to a triangular straight-line gap, not every possible curve shape. Label the social curves and the quantity interval before shading; a rectangle for all external damage is not the welfare-loss triangle.

Compare decisions by net social benefit

For a discrete project, compare total social benefit with total social cost. This differs from choosing a continuous output by equality of marginal values. Net present value is outside the required scope here.

Worked example. Projects A and B are mutually exclusive. All values cover the same year and include all relevant private and external effects.

Project	SB (£/year)	SC (£/year)
A	1600	1000
B	1900	1500

A has the larger net social gain despite B's higher gross benefit. If A's social cost estimate rises to £1300/year:

B is then preferred on these estimates. Check common time periods, opportunity costs, omitted effects and double counting. Consider who gains or loses and how reliable the valuations are. A positive total net gain does not establish a Pareto improvement.

Asymmetric information and moral hazard

Asymmetric information 信息不对称 means one party has relevant information the other lacks. For example, a seller knows a car has a hidden defect that the buyer cannot assess. This can distort the buyer's valuation and choice. It is a particular information imbalance, not every case in which both parties lack knowledge.

Moral hazard 道德风险 occurs when bearing less of an action's risk weakens the incentive to take care and can encourage risk-taking. An insured operator may become less careful because the insurer bears most accident costs. Identify the changed incentive, not just the existence of insurance.

The mechanisms can coexist, but a hidden defect alone does not establish increased risk-taking. Harm to a third party alone establishes neither unequal information nor a changed risk incentive. Explain the mechanism before selecting the label.

Transfer to a case with both cost and benefit spillovers

Check yourself. Let $MPB=10-Q$, $MPC=2+Q$, $MSB=14-Q$ and $MSC=4+Q$, using numerical output in units/month and marginal values in £/unit. A student predicts excess output simply because MSC exceeds MPC. Test the prediction.

Worked answer. Under the competitive private-market conditions:

For the social optimum:

The market underprovides: the external-benefit wedge exceeds the external-cost wedge. Use both social curves. A single-wedge rule cannot decide a mixed case; depending on the relative effects, the market could produce too much, too little or the same quantity as the social optimum.

Short-run production: total, average and marginal product

In the short run 短期, at least one factor of production is fixed; in the long run 长期, all factors can change. These are economic conditions, not fixed numbers of months. A production function relates combinations of inputs to the output obtainable with a given technology.

Worked example. A bakery keeps one oven and the same technology. It changes the number of workers. Total product 总产量 (TP) is the output per day. Average product 平均产量 (AP) is output per unit of the variable input. Marginal product 边际产量 (MP) is the extra output per extra unit of that input.

Workers L	TP (loaves/day)	AP (loaves/worker/day)	MP (loaves/worker/day)
0	0	undefined	—
1	12	12	12
2	30	15	18
3	42	14	12
4	48	12	6
5	50	10	2
6	48	8	−2

For two workers:

For the second worker:

The second worker adds more than the first, perhaps because tasks can be shared. From the third worker, MP falls: extra workers must share the same oven. The law of diminishing returns 边际报酬递减 says that, with at least one input fixed and technology unchanged, extra units of the variable input eventually add less output. It does not say that total output immediately falls. TP rises while MP is positive; TP falls when MP is negative. Zero MP means no change in TP over that interval.

MP above AP pulls AP up; MP below AP pulls AP down. In a smooth model, MP crosses AP at AP's maximum. A discrete table need not contain exact equality: here AP peaks at two workers. Diminishing returns is a short-run change in the input mix; returns to scale changes all inputs together in the long run.

Short-run costs: totals, averages and changes

A fixed cost 固定成本 does not change with output in the chosen short-run period. A variable cost 可变成本 changes with output. Total fixed cost (TFC) and total variable cost (TVC) add to total cost 总成本 (TC). Fixed does not mean sunk: a sunk cost cannot be recovered, whereas a fixed cost is defined by its relationship with output.

Worked example. The bakery pays £60/day for its oven at every output, including zero. The following is a separate cost schedule, not a claim that it is calculated from the preceding worker table.

Q (loaves/day)	TFC (£/day)	TVC (£/day)	TC (£/day)	MC (£/loaf)
0	60	0	60	—
10	60	40	100	4
20	60	60	120	2
30	60	90	150	3
40	60	140	200	5

At 30 loaves/day:

For the increase from 20 to 30 loaves/day:

This is the average additional cost of that ten-loaf block; the table does not observe each individual loaf's marginal cost. Since TFC is unchanged, the change in TC equals the change in TVC.

Average fixed cost 平均固定成本 (AFC), average variable cost 平均可变成本 (AVC) and average total cost (ATC) use the same output denominator. Here AC means ATC. At 20 loaves/day:

Q (loaves/day)	AFC (£/loaf)	AVC (£/loaf)	ATC (£/loaf)
10	6	4	10
20	3	3	6
30	2	3	5
40	1.5	3.5	5

AFC continually falls as positive output increases because the same fixed cost is spread over more output. ATC need not continually fall: AVC can rise. Here the last block's MC equals the preceding ATC, so ATC is unchanged. The lowest observed ATC does not prove the minimum at every possible output. At zero output, none of these averages is defined.

Explain cost curves using productivity and marginal changes

With a fixed wage and only labour changing as a variable input, labour's additional cost per unit of output is wage divided by positive MP. If an extra worker costs £60/day and adds 12 loaves/day:

If an extra worker adds only 6 loaves/day:

Thus falling positive MP can raise MC when the wage is unchanged. Other changing input costs must also be counted. The usual smooth short-run model has initially increasing productivity and later diminishing returns, giving U-shaped AVC and MC; this is a model, not a rule that every observed cost schedule must have exactly that shape.

An additional unit costing less than the current average pulls the average down; one costing more pulls it up. In a smooth differentiable model, MC equals AVC at AVC's minimum and ATC at ATC's minimum. The two minima generally occur at different outputs.

Transfer question. A firm sells 10000 units/month at £10/unit. TFC is £40000/month and the firm makes only normal profit. Find AVC, including normal profit in economic costs.

Worked answer. Normal profit means zero economic profit, so TC equals TR:

Here AR equals ATC, so subtracting AFC from price also gives AVC. That shortcut would fail if the firm earned supernormal or subnormal profit.

Long-run costs and scale

In the long run, no factor of production is fixed. Returns to scale 规模报酬 compare output when all inputs change in the same proportion, with technology unchanged. This differs from diminishing marginal returns, which changes the variable input while at least one other input remains fixed.

Worked example. A firm uses 100 workers and 2 machines to produce 1000 units/day. It doubles both inputs to 200 workers and 4 machines, producing 2500 units/day.

This is increasing returns to scale 规模报酬递增: output rises more than proportionately. With the same doubling of inputs, an output of 2000 units/day would show constant returns to scale 规模报酬不变; 1500 units/day would show decreasing returns to scale 规模报酬递减. These are alternative hypothetical results, not extra observations of this firm.

Physical returns and cost changes are linked only with suitable assumptions about input prices. Suppose the input bundle costs £10000/day initially and £20000/day after doubling, at unchanged input prices:

Economies of scale 规模经济 lower long-run average cost as scale increases. Here cost doubles while output rises by 2.5 times. If input prices change, the physical comparison alone does not determine the cost change. Compare the same product quality and technology; a cost reduction caused by new technology alone is not automatically caused by scale.

Long-run average cost (LRAC) is the lowest attainable average cost at each output when all inputs can be chosen. Minimum efficient scale 最小有效规模 (MES) is the smallest output at the minimum LRAC. It need not be the largest output shown, and a flat minimum can contain several equally efficient scales.

Worked example. Treat the following listed outputs as the available choices:

Output (hundreds of units/day)	LRAC (£/unit)
1	12
2	9
3	8
4	8
5	9

The minimum cost is £8/unit at both 300 and 400 units/day. MES is the first of these, 300 units/day. The following illustrative smooth model also has a flat minimum from 300 to 400 units/day. Diseconomies of scale 规模不经济 raise LRAC as scale increases beyond that range.

Internal and external scale effects

Classify the effect by whose expansion causes the cost change, then explain the mechanism:

• Internal economies of scale 内部规模经济 arise from the firm's own expansion. A specialised machine or division of tasks can increase output per resource. Bulk purchasing or cheaper finance can reduce input costs. The gain must lower cost per unit, not merely increase total output.
• External economies of scale 外部规模经济 arise from expansion of the industry. A larger local industry can support specialist suppliers, repair services or a pool of trained workers. A firm can gain even if its own size is unchanged.
• Internal diseconomies of scale 内部规模不经济 arise from the firm's expansion. More management layers, communication failures and weaker coordination can raise its unit cost.
• External diseconomies of scale 外部规模不经济 arise from industry expansion. Competition for local sites or skilled labour can bid up rents and wages; congestion can raise transport costs for several firms.

Internal/external here describes the source of a scale effect. It does not mean private/social cost, and a benefit to a neighbour is not automatically an external economy of scale: industry expansion must cause the cost reduction.

Spreading an unchanged fixed cost over more output is a short-run AFC effect, not by itself evidence of a long-run scale economy. AFC can fall while rising AVC causes ATC to rise. Long-run comparisons allow every input to change. Lower total cost alone proves neither lower average cost nor greater efficiency at a different quality of output.

Read average cost from long-run total cost

On a total-cost diagram, average cost is total cost divided by output: the slope of a straight ray from the origin to the point on the total-cost curve. The local slope of the curve instead measures marginal cost in a smooth model. Do not choose the flattest part of LRTC when asked for minimum LRAC.

Worked example. The following separate model has LRTC values of £1200, £1800, £2400 and £3600/day at 100, 200, 300 and 400 units/day. Divide each total by its own output to obtain LRAC values of £12, £9, £8 and £9/unit.

Revenue: separate totals, averages and changes

Total revenue 总收益 (TR) is all revenue from sales. Average revenue 平均收益 (AR) is revenue per unit sold. Marginal revenue 边际收益 (MR) is the additional revenue per extra unit of output. When the firm charges one price for all units:

Worked example. A firm sells 10 units/day at £50/unit. To sell 11 units/day it must lower the price to £48/unit on every unit.

The extra unit brings £48, but the price cut loses £2 on each of the previous ten units. Net revenue rises by only £28/day. MR is not the new selling price.

Contrast. A price taker can sell either 10 or 11 units/day at an unchanged £50/unit. With no price cut on earlier sales:

An unchanged price gives MR=AR=P. With downward-sloping demand and a single price, MR is generally below AR because selling more requires a lower price on existing sales. With price discrimination, calculate actual total receipts; one common price cannot be assumed. Average revenue at zero sales is undefined.

Profit: include the opportunity cost of the owner's resources

Economic profit 经济利润 is total revenue minus all economic costs, including the opportunity cost of the owner's time and funds. Normal profit 正常利润 is the minimum required return that keeps the entrepreneur's resources in this use. It is counted within economic cost. Supernormal profit 超常利润 is profit above normal; subnormal profit 次正常利润 is a return below normal.

Worked example. Annual revenue is £24000 and explicit costs are £18000. The owner's time and funds have an implicit opportunity cost of £6000/year, with no other costs in this example:

The firm makes normal profit: its required return is covered, so economic profit is zero. Accounting profit is £6000/year here because it omits that implicit cost. Zero economic profit does not mean the owner receives nothing.

For positive output, profit can also be calculated from the gap between average revenue and average total cost:

Worked example. Keep output at 100 units/month and economic AC at £9/unit across three alternative revenue conditions:

These are supernormal, normal and subnormal profit respectively. Profit is the total gap over the period, not just the per-unit AR−AC difference.

Compare feasible output choices using profit

Try it. Only the following outputs are feasible in a separate daily example. Calculate profit for each row, then identify the best choice.

Q (units/day)	TR (£/day)	TC (£/day)	Profit (£/day)
0	0	4	−4
1	10	8	2
2	18	13	5
3	24	19	5
4	28	26	2

Worked answer. Outputs 2 and 3 both maximise profit at £5/day. Increasing output from 2 to 3 gives:

Across the successive one-unit steps, MR is £10, £8, £6 and £4/unit; MC is £4, £5, £6 and £7/unit. Profit rises when the extra revenue exceeds extra cost, is unchanged when they are equal, and falls when MC exceeds MR. The last unit raises total revenue but reduces profit. A discrete table can have a tie, so a unique output cannot always be inferred from MR=MC.

Profit levels and output decisions answer different questions

AR=AC means zero economic profit. It does not identify the output with the largest profit. MR=MC is a marginal condition: for a smooth interior maximum, marginal profit must change from positive to negative. Check feasible boundaries and stopping production too. With several intersections, compare total profit rather than assuming every MR=MC point is a maximum.

Worked example. In the graph, AR=14−Q, MR=14−2Q and MC=2+2Q, in £/unit, where Q is the numeric output in units/day. Total cost is £(10+2Q+Q²)/day. Marginal revenue exceeds marginal cost below 3 and falls below it above 3:

At 3 units/day, price is £11/unit, total revenue £33/day and total cost £25/day, giving profit £8/day. At either 1 or 5 units/day, revenue equals cost and profit is zero. This shows why a normal-profit question asks for the AR/AC intersection, not the MR/MC intersection.

A short-run loss and the decision to stop production

A negative economic profit does not automatically mean immediate closure is best. Some fixed costs may be unavoidable for the current period. Compare the loss from producing with the loss from stopping; keep long-run exit separate.

Worked example. Only 0 or 100 units/month are feasible this month. At 100 units, AVC is £4/unit and ATC is £6/unit. Fixed cost is £200/month and unavoidable; variable costs can be avoided by stopping. At a price of £5/unit:

Stopping gives no revenue and no variable cost, leaving the £200/month fixed-cost loss. Producing is preferable at this price because it covers variable cost and contributes to fixed cost. At a price of £3/unit:

Now stopping loses less. At a price equal to AVC, producing and stopping give the same loss under these assumptions. With more output choices, compare their contributions too. Long-run exit allows a new choice of all resources: reconsider avoidable costs, alternative opportunities and expected future returns rather than treating current fixed costs as unavoidable for ever.

A market structure 市场结构 describes the number and size of sellers, entry conditions and the product sold. Define the product and geographical market before counting firms: a cafe may have no rival on one street but face rivals within walking distance.

Structure	Sellers	Product	Entry conditions
perfect competition 完全竞争	very many	identical	free entry and exit
monopolistic competition 垄断竞争	many	differentiated	relatively easy
oligopoly 寡头垄断	a few dominate	identical or different	substantial barriers
monopoly 垄断	one	no close substitute	protected entry

Product differentiation 产品差异化 means buyers see differences in quality, location, branding or service. It can give a firm some price-setting power. Barriers to entry 进入壁垒 include legal restrictions, difficult access to key inputs and cost advantages that a new supplier cannot readily match. They can help profit persist, but cannot guarantee profitable demand. A monopoly can make normal profit or a loss; an oligopoly need not form a cartel 卡特尔. Collusion 合谋 means firms coordinate decisions instead of acting independently. A cartel is an agreement between otherwise separate firms to restrict supply or fix prices. Potential competition must also be considered: a small number of current sellers does not by itself establish freedom from competitive pressure.

Worked classification. Many small farms selling the same grade of wheat at the market price fit perfect competition most closely. Many cafes with different menus and easy entry fit monopolistic competition. Three airlines that watch each other's fares and face expensive entry fit oligopoly. These are illustrative models, not claims that real markets meet every assumption. Reported profit alone does not identify structure.

Perfect competition: distinguish firm and market

A small price taker 价格接受者 accepts a market price that its own sales do not change. Market demand and supply determine this price. The individual firm's demand is horizontal at it; the whole market's demand curve is generally downward sloping.

At £10/unit, the firm sells 4 or 5 units/day:

Thus $P=AR=MR$. Unlike the earlier price-cut example, an extra sale does not reduce the price received on previous sales. Find the best feasible output using marginal and total comparisons, not from the firm's name.

Short-run profit and shutdown

Worked comparison. In each separate case below, the best positive output is supplied as 4 units/day, TC is £40/day and TVC is £24/day. These are supplied case data, not a claim that a firm's optimum never changes when price changes. Use $TR=PQ$ and $\pi=TR-TC$.

Price (£/unit)	TR (£/day)	TC (£/day)	Profit (£/day)
12	48	40	8
10	40	40	0
8	32	40	−8

All three outcomes are possible for a competitive firm in the short run. Zero economic profit includes the normal return on the owner's resources. Unavoidable fixed cost and average variable cost are:

At £8/unit, production covers TVC and contributes £8/day towards fixed cost. The £8/day loss is smaller than the £16/day loss from stopping. In general compare the best positive-output contribution with stopping, allowing for which costs are actually avoidable. Do not use a short-run shutdown rule as an automatic long-run exit decision.

Entry, exit and the long-run competitive equilibrium

With free entry, economic profit attracts new firms. Market supply increases, tending to lower price and existing firms' profits. Losses encourage exit; reduced supply tends to raise price. This adjustment takes time. The familiar long-run result assumes unchanged conditions, informed buyers, access to the required inputs and technology, and no effective barriers to entry or exit. It is not a prediction that every observed farm earns exactly normal profit today.

The diagram uses $ATC=10+0.3(Q-5)^2$ and the consistent $MC=d(Q\times ATC)/dQ=17.5-6Q+0.9Q^2$. Costs are £/unit and output is units/day. At $Q=5$, ATC and rising MC are both £10/unit; the other lower-output MC intersection is not the maximum-profit choice. For this illustration the total-cost model is $TC=17.5Q-3Q^2+0.3Q^3$; it is separate from the earlier fixed-cost examples.

Producing at minimum ATC is productive efficiency for the given technology and input prices. The equality $P=MC$ implies allocative efficiency only when price represents marginal benefit and private and social margins coincide. External effects or poor information can break this welfare conclusion. Neither normal profit nor a cost minimum alone proves dynamic efficiency: judge investment, finance and innovation incentives separately. Under the standard exam model, the absence of sustained supernormal profit can limit internal finance for research; this does not mean no competitive firm ever innovates.

Derive a firm's short-run supply

Worked discrete comparison. A different firm can produce only 0,1,2,3,4 units/day. TFC is £6/day, unavoidable in the short run; TVC is £0,£2,£6,£12,£20/day. Successive one-unit marginal costs are therefore £2,£4,£6,£8/unit. Compare all feasible outputs, including zero, at each price using $\pi=PQ-(TFC+TVC)$.

Price (£/unit)	Best Q (units/day)	TR (£/day)	TC (£/day)	Profit (£/day)
1	0	0	6	−6
3	1	3	8	−5
5	2	10	12	−2
7	3	21	18	3

At £3, selling a second unit earns £3 but costs £4; adding it lowers profit. At £7, the fourth unit costs £8 and earns £7. Price below £2 leads to zero output; at £2 the firm is indifferent between zero and one unit. At prices £4,£6 and £8 the corresponding adjacent outputs tie. Do not force an equality rule on discrete steps when price lies between marginal costs.

For a conventional smooth cost model, the firm's short-run supply is the rising part of MC above minimum AVC. Below minimum AVC it supplies zero. At that minimum price it is indifferent between stopping and the best positive output. This assumes the indicated branch gives the best feasible contribution and fixed costs are unavoidable. Add the quantities supplied by firms at the same price to obtain industry supply. A monopoly's optimum depends on demand and MR as well as costs, so it has no equivalent price-only supply curve.

Monopoly: choose output, then read price and profit

A monopoly faces the market demand curve. An extra sale normally requires a lower price, so MR is below AR on the downward-sloping demand curve. Choosing a price does not let the firm sell any quantity it wants: its price and sales must be consistent with demand.

Worked model. Use $AR=P=18-Q$, $MR=18-2Q$ and $TC=20+2Q+Q^2$, with the consistent $MC=2+2Q$ and $ATC=20/Q+2+Q$. Output is units/day, price and marginal costs are £/unit and total cost is £/day. These numerical equations are an illustrative model, separate from the earlier competitive firm's cost curves.

At a smooth interior profit maximum:

Read price from AR, not MR:

The marginal difference $MR-MC=16-4Q$ is positive below 4 and negative above it. Thus the firm gains by expanding towards 4 from below and loses by expanding beyond 4. This quadratic profit model has a single positive-output maximum. Stopping loses the £20/day fixed cost if it is unavoidable; producing is better. For other cost/demand shapes, check all relevant branches, boundaries and feasible total profits.

The gap between price and MC is not profit per unit. It excludes some costs; use price minus ATC instead.

Entry barriers do not guarantee supernormal profit

Keep this demand and variable cost $TVC=2Q+Q^2$ unchanged but compare three alternative unavoidable fixed costs. The same MC implies the same optimum $Q=4$ and $P=£14$/unit in these particular cases.

TFC (£/day)	TR (£/day)	TC (£/day)	Profit (£/day)
20	56	44	12
32	56	56	0
40	56	64	−8

At this output, variable cost is £24/day in every case. Changing unavoidable fixed cost shifts ATC and profit, but not MC here. The first monopoly earns supernormal profit, the second normal profit, and the third a loss. In the third case producing loses £8/day, whereas stopping loses £40/day, so production is preferable in the short run. If the cost becomes avoidable, or future demand changes, compare the long-run options again. Barriers can prevent entrants from competing a profit away; they cannot create sufficient demand or remove a firm's costs.

Compare profit maximisation with the social margin

With no external effects or information problems, suppose the demand price represents marginal social benefit and MC represents marginal social cost. At the monopoly output of 4, buyers value a small extra unit at £14 while it costs £10 to supply. Extra output can therefore add net social benefit even though it reduces the monopoly's profit.

The illustrative social optimum uses $MSB=MSC$, not $MR=MC$:

This compares outcomes using unchanged demand and costs, without other distortions. It is not a claim that every price control will work: a real policy must consider information, cost recovery, investment incentives and implementation. Nor does monopoly status prove poor innovation or inefficient management; investigate costs, incentives and evidence.

Monopolistic competition: entry, normal profit and excess capacity

Many sellers offer differentiated products, with relatively easy entry and exit. Buyers may not know every quality difference or price. Each firm faces a downward-sloping demand curve because it can lose customers to close substitutes; some price-setting power does not mean unlimited freedom to raise price. In the short run, demand and costs can give profit or loss. In the standard long-run model, profit attracts entry of close substitutes, reducing each incumbent's demand until economic profit is zero. Losses encourage exit. The remaining firm's demand does not become horizontal merely because entry is free.

Worked long-run illustration. Keep $AR=18-Q$, $MR=18-2Q$ and $MC=2+2Q$, but use $TC=32+2Q+Q^2$. At $Q=4$ units/day, $MR=MC$ and price read from AR is £14/unit:

Here $ATC=32/Q+2+Q$. At output 5 it is £13.40/unit, already below £14/unit at 4; the minimum is about £13.31/unit at output 5.66. Thus the firm has excess capacity relative to the output minimising ATC in this model. Also $P=14>MC=10$, so the private marginal condition for allocative efficiency is not met. Free entry and normal profit do not make this the same outcome as perfect competition. Product variety may benefit buyers; weigh that benefit rather than assuming that identical products must always be best.

Natural monopoly and the relevant cost range

A natural monopoly 自然垄断 exists when one firm can supply the relevant market at lower total cost than multiple firms. A large network cost and falling LRAC over the range of market demand can produce this condition. It is a cost argument, not simply a statement that one seller happens to exist.

Worked network. Market demand is 100 identical service units/month. Each active network requires £120/month of overhead plus £2/unit supplied. Treat these as the least feasible costs of a network in this illustration, allowing a provider to close; two providers require two separate networks. Quality and input prices are identical.

Duplicating the network adds £120/month. Over this relevant demand range, one provider has the lower total cost. This does not justify any price it chooses or prove good management. A larger market, different technology or shared network access may change the comparison. Regulation must consider service standards, cost recovery and investment as well as price.

X-inefficiency: avoidable cost at a given output

X-inefficiency X低效率 means avoidable costs above a feasible best-practice cost for the same output. Weak pressure to control costs may permit waste, poor supervision or slack management, but neither monopoly status nor a high observed cost proves this on its own.

Worked comparison. A network supplies 100 service units/month at an actual cost of £440/month. A feasible best-practice method costs £320/month, with the same input prices, quality and reliability:

A costly remote location or higher quality might instead explain higher costs without avoidable inefficiency. Control those differences before drawing the conclusion. Producing away from minimum ATC concerns output choice along a cost curve. X-inefficiency concerns avoidable cost above the feasible curve at a given output. These are different tests.

Concentration ratios: combined shares of the largest firms

A concentration ratio 集中度 is the combined market share of a specified number of the largest firms. Define the product, geographical market, period and measure, such as sales revenue. Use a consistent measure in both numerator and denominator.

Worked four-firm ratio. Firms A–F have annual sales of £60m, £50m, £40m, £20m, £16m and £14m. Total market sales are £200m/year. The largest four have £170m/year:

Now A and B merge and all sales stay unchanged. The merged firm has £110m/year, followed by firms with £40m, £20m, £16m and £14m. Re-rank before selecting the largest four:

All numerator and denominator values use £million/year. The increase is 8 percentage points. Do not count A and B separately after they merge; do not confuse an 85% combined share with 85% for each firm. High concentration shows dominance by a few large sellers but does not alone prove collusion, high prices or weak potential entry.

Entry, exit and contestability

Explain how a barrier works. A restrictive licence is a legal barrier. A scarce suitable harbour site is a physical obstacle. An incumbent's large low-cost network may give a cost advantage that a smaller entrant cannot match. Strong buyer loyalty to an established brand is a market obstacle. Categories can overlap; the mechanism matters more than assigning a single label. Perfect competition assumes many buyers and sellers, informed choices, an identical product and free entry and exit. Few sellers with imperfect information may have a different competitive position even when other features look similar.

A contestable market 可竞争市场 faces effective pressure from potential entrants. In the ideal model, entry and exit are free, entrants can access comparable technology and costs, and there are no unrecoverable entry costs. Even a few current sellers may keep prices close to cost if a newcomer can quickly win business and leave without a large loss. The number of current firms is therefore not a complete test of competition. A temporary profit opportunity is less threatening if entering takes years or incumbents can react before entrants recover their investment.

A sunk cost 沉没成本 is unrecoverable once incurred. It is different from a fixed cost, which describes how cost varies with output during a period. Equipment that can be sold on exit is not wholly sunk.

Worked entry investment. A newcomer spends £20,000 on equipment and £5,000 on launch advertising. It can resell the equipment for £18,000 on exit; the advertising cannot be recovered. Ignoring other costs and receipts:

This £7,000 makes quick entry and exit less attractive than the full-recovery case. Once already incurred, however, sunk cost should not itself justify continued production: compare future avoidable costs and returns. Future exit penalties and obligations may affect that comparison. Do not claim that past sunk spending automatically traps a firm into rationally operating at a continuing avoidable loss.

Oligopoly: interdependent choices and a payoff matrix

In oligopoly, a few firms dominate and each firm's outcome depends on how rivals respond. Firms may compete on price, service or quality, or coordinate decisions. Neither collusion nor a price war is inevitable. A payoff matrix 收益矩阵 lists each player's outcome for every combination of choices.

Illustrative one-shot game. Two firms independently and simultaneously choose high or low prices, without a binding agreement. Each cell gives (A's profit, B's profit) in £/week. These are teaching data, not a prediction of actual airline or cafe profits.

A's choice	B: high	B: low
High	(700, 700)	(200, 900)
Low	(900, 200)	(400, 400)

Read the first number for A and the second for B. If B chooses high, A prefers low: £900/week rather than £700/week. If B chooses low, A still prefers low: £400/week rather than £200/week. The symmetric reasoning gives the same preferred responses for B. Thus both choose low, making £400/week each. Neither can improve by changing alone from low/low.

Yet high/high would give £700/week each. This is a Prisoner's Dilemma 囚徒困境: each firm's individually preferred response produces an outcome worse for both than mutual cooperation in this table.

The higher joint total does not remove the temptation for one firm to undercut while the other keeps a high price. A cartel must address incentives, monitoring and responses to cheating. New entrants, differing costs and many participants can weaken coordination. Repeated dealings and credible future responses may change incentives; the one-shot result is conditional on the displayed payoffs and assumptions. Do not infer one universal outcome for every oligopoly.

Price and non-price competition: compare profits, not only sales

Price competition 价格竞争 means changing price to attract buyers. Non-price competition 非价格竞争 uses quality, service, branding or promotion. Advertising can communicate useful differences or build loyalty; better service may increase costs as well as demand. Rivals may imitate either approach, so neither is guaranteed to succeed.

Worked monthly forecasts. Initially price is £10/unit, sales are 100 units/month, unit variable cost is £6 and fixed cost is £200/month. A price cut to £9 is forecast to sell 115 units/month with the same fixed cost. Alternatively, price stays £10 and extra service/advertising costing £60/month is forecast to raise sales to 120 units/month. Use $TR=PQ$, $TC=TFC+vQ$ and $\pi=TR-TC$, where $v=£6$/unit.

Case	Q	TR	TC	Profit
Initial	100	1000	800	200
Price cut	115	1035	890	145
Service/ad	120	1200	980	220

Output is units/month; money totals are £/month. The price cut increases revenue by £35/month but reduces profit by £55/month. The service/advertising forecast increases profit by £20/month after its extra cost. For example:

These are forecasts. If rivals match the offer or demand disappoints, recalculate rather than assuming the stated sales increase. A price reduction may benefit buyers even if it reduces the firm's profit.

Price discrimination: distinguish the pricing rule

Price discrimination 价格歧视 means different prices for the same product where the difference is not explained by different supply costs. A higher delivery charge for a distant address may reflect a higher cost, so different prices alone do not prove discrimination. A ticket bought early and a ticket bought just before departure may sort travellers by their willingness to pay; changing demand or capacity also needs consideration. Do not infer the precise degree from timing alone.

• First-degree price discrimination 一级价格歧视 charges each unit at the individual buyer's maximum willingness to pay for that unit. It requires detailed information and control over resale; it is an idealised benchmark, not simply charging one high price to everyone.
• Second-degree price discrimination 二级价格歧视 varies the unit price with quantity or a menu of purchase options. Buyers select an option rather than the seller assigning a price from their identity. A quantity discount is discriminatory only insofar as its price differences are not explained by cost differences.
• Third-degree price discrimination 三级价格歧视 charges different identifiable groups different prices for the same product, for example an ID-checked student ticket and an adult ticket. Groups must be kept separate. Different demand responses allow different profit-maximising mark-ups; it is not necessary for every group to have inelastic demand.

Compare a uniform price with first-degree pricing

Illustrative three-unit model. One buyer is willing to pay up to £12 for the first unit, £9 for the second and £6 for the third. Additional units have no value to this buyer. Each unit costs £4 to supply, there is no fixed cost, and the buyer purchases when willingness to pay is at least the price. Ignore administration costs initially.

Uniform price (£/unit)	Units bought	TR (£)	Cost (£)	Profit (£)
12	1	12	4	8
9	2	18	8	10
6	3	18	12	6

These willingness-to-pay thresholds give the best uniform prices for each possible positive quantity. A higher price within a range cannot sell more; lowering it within that range only reduces revenue. Not selling gives zero profit. The best uniform choice is therefore £9/unit and two units, with profit £10.

Under first-degree pricing the seller charges £12, £9 and £6 for the three units. All three units' marginal values here exceed the £4 marginal cost:

The uniform-price buyer keeps £3 of consumer surplus on the first unit. Perfect first-degree pricing takes that surplus and also serves the third unit. Consumer surplus is zero under the stated purchase-at-indifference assumption. With marginal willingness to pay representing social benefit, marginal cost representing social cost and no other distortion, supplying the third unit adds £2 of total surplus. This efficiency result does not establish a fair distribution or that perfect information is realistic.

Second-degree pricing: let the buyer choose a quantity

Keep the same buyer and constant £4/unit cost. Offer the first two units at £9 each and the third at £5. The total bill is £9 for one, £18 for two and £23 for three. The buyer's total willingness to pay is £12, £21 and £27 respectively. Net benefits are £3, £3 and £4, so the buyer strictly prefers three units. This is a quantity menu, not a price based on a student ID.

Profit is £1 above the best uniform-price profit. Extra administration cost above £1 removes that gain. The constant cost assumption makes clear that this price pattern is not simply passing on a cheaper cost of supplying the third unit. A different buyer may choose another quantity; the menu does not force everyone to buy three.

Third-degree pricing: separate identifiable groups

A separate illustrative market. Each customer buys at most one visit; cost is £4/visit, with no fixed or administration cost. In group A two customers will pay up to £12 and £10. In group B two will pay up to £7 and £5. Customers buy at a price equal to their maximum willingness to pay. Group membership can be verified and tickets cannot be transferred.

One price (£/visit)	Visits sold	TR (£)	Profit (£)
12	1	12	8
10	2	20	12
7	3	21	9
5	4	20	4

The best uniform price is £10, giving two visits and £12 profit. For separate group prices, A's best choice is £10 for two visits, giving £12 profit. B's best choice is £7 for one visit, giving £3 profit, rather than £5 for two visits giving £2 profit. Together:

One extra B customer is served and profit rises by £3 before administration costs. This numerical result does not imply that all customers gain or that discrimination always expands output. The values are illustrative thresholds, not estimates of demand elasticities. In the usual smooth third-degree model with a common marginal cost, the group with less elastic demand at its chosen price bears a higher mark-up. Apply that rule to the relevant demand response rather than assuming age or income alone proves elasticity.

Conditions and consequences: test the mechanism

Effective discrimination requires some price-setting power, information that allows useful prices or options to be set, and limits on resale or switching between prices. Third-degree group pricing additionally needs identifiable, separable segments with different demand responses. A full single-seller monopoly is not required; price leadership is not a necessary condition. ID checks, non-transferable services or quantity menus may help, but verification costs and customer behaviour matter.

If any adult can buy a transferable student ticket and resell it, the higher-price group can bypass the adult price. Cheap resale undermines separation. An ID check may reduce that route, but its cost and enforcement determine whether it works. A firm that guesses willingness to pay badly can lose buyers; the original uniform price remains an available alternative only if the firm can choose it and avoid extra costs.

Compare the whole outcome with a feasible alternative. Some buyers may pay more, others may gain access at a lower price. Output can rise, fall or remain unchanged, so the effect on allocative efficiency is conditional. Perfect first-degree pricing can remove the restricted-output loss under the benchmark assumptions above while transferring consumer surplus to the firm. Third-degree pricing does not in general remove that loss. Higher profit might finance investment or keep a service operating, but neither follows automatically. Administration costs, information limits, resale and entry can reduce or remove the gain. Judge profit, access, efficiency and distribution separately.