Project management: the critical path is the project
Introduit| English | Sens |
|---|---|
| critical path/ˈkrɪtɪkl pæθ/ | The longest chain of linked tasks in a project; a delay on it delays the whole project. |
| total float/ˈtəʊtl fləʊt/ | how long an activity can slip without delaying the whole project |
| crashing | shortening critical activities by spending money on them |
| earned value analysis | tracking a project with planned, actual and earned value and the ratios between them |
Eighteen days of planned work
- The second-site fit-out as activities: A design 2, B strip-out 3, C rewire 5, D oven 4, E ventilation 4, F decor 3, G test 2, H handover 1.
- Plan, crash and track are three disciplines that keep a project from being a story told afterwards.
The network and its critical path
- Forward pass: A-B-C-D-F-H = 2 + 3 + 5 + 4 + 3 + 1 = 18 days. That chain is the critical path 关键路径: zero float on every activity.
- Backward pass: E can finish by 14 but finishes at 9 - total float 总时差 5. G can finish by 17, finishes at 16 - float 1.
A 2, B 3, C 5, D 4, F 3, H 1 in sequence. The project duration in days?
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2 + 3 + 5 + 4 + 3 + 1 = 18.
E (4 days) can start at day 5 and must finish by day 14. Its total float?
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Late finish 14 minus early finish 9 = 5.
Crashing: buy days where the path runs
- Crashing 赶工 C by 2 days costs 400 a day; D by 1 day costs 600. A day saved is worth 700 at the festival deadline.
- C first: 16 days, 800 spent for 1,400 of value; the G branch is now 15, so one more day on D pulls both to 15 for 600 against 700. Total: 18 to 15 days, 1,400 spent, 2,100 earned, net +700.
- Float is the project's shock absorber, not a trade's slack: release it only in writing, with the end date recomputed.
Critical, floating, or worth crashing?
Sort each activity or crash offer by what the network says about it.
Which crash first, and why?
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Only critical days shorten the project; C is on the path and cheapest per day.
Float belongs to the trade whose activity carries it.
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Float is the project's shock absorber - released only in writing with the end date recomputed.
Earned value: two ratios, early
- Budget 60,000 (the raise of course 9). Day 10: planned value 33,000, actual cost 36,400, earned value 30,000 (half the scope).
- Earned value analysis 挣值分析: $CPI = 30{,}000/36{,}400 = 0.824$, $SPI = 30{,}000/33{,}000 = 0.909$; $EAC = 60{,}000/0.824 = 72{,}800$ - a 12,800 overrun visible on day 10, while it is still a decision.
EV 30,000 and AC 36,400. The CPI (three decimals)?
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30,000 / 36,400 = 0.824.
BAC 60,000 and CPI 0.824. The estimate at completion?
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60,000 / 0.824 = 72,800 (exactly 60,000 x 36,400 / 30,000).
SPI = 0.909 means:
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EV/PV below 1: less value earned than planned by now.
Match each earned-value term to its day-10 figure.
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Two ratios come from these: CPI = EV/AC, SPI = EV/PV.
Three project slips
- Crashing the cheapest activity instead of a critical one. Money off the path buys nothing.
- Letting trades consume float. It belongs to the project.
- Tracking spend without earned value. 61% of the money is not 61% of the work.
Recap
- Critical path A-B-C-D-F-H = 18 days; floats E 5, G 1.
- Crash where the path runs, below the day's value: 18 to 15 for net +700.
- CPI 0.824, SPI 0.909, EAC 72,800 - flagged on day 10.