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Difference between CPM and PERT
Short answer
CPM uses a single deterministic time estimate for activities, is activity-oriented, and suitable for repetitive projects. PERT uses three time estimates (optimistic, most likely, pessimistic) to calculate expected time and variance probabilistically, and is event-oriented for uncertain projects.
What is the difference between CPM and PERT?
CPM (Critical Path Method) and PERT (Program Evaluation and Review Technique) are network-based scheduling techniques that identify the critical path. CPM was developed in 1957 (DuPont) for construction/industrial projects; PERT was developed in 1958 (US Navy, Polaris missile) for R&D projects.
| Basis | CPM | PERT |
|---|---|---|
| Time estimate | One deterministic estimate | Three estimates: optimistic (a), most likely (m), pessimistic (b) |
| Nature | Deterministic | Probabilistic |
| Expected time | Single duration given | te = (a + 4m + b) / 6 |
| Variance | Not considered | σ² = ((b − a)/6)² |
| Orientation | Activity-oriented | Event-oriented |
| Use | Repetitive, well-known projects (construction, maintenance) | Non-repetitive, uncertain projects (R&D, new software) |
| Cost | Considers time-cost trade-off (crashing) | Mainly time; cost is secondary |
| Probability | Cannot find probability of completion | Finds probability of completing by a date using normal distribution |
| Focus | Controlling cost | Controlling time |
| Network | Activity on arrow or node | Mostly activity on arrow |
| Distribution | None | Beta distribution assumed |
Common points
Both use network diagrams, identify critical path and float, need a WBS and activity dependencies, and support planning and control.
Example of PERT numbers
a = 4, m = 6, b = 14 days: te = (4 + 24 + 14)/6 = 7 days; σ = (14 − 4)/6 = 1.67 days.
CPM: Activity -> single time -> critical path
PERT: Activity -> a, m, b -> te, variance -> probability
Modern software often uses CPM-style calculations with PERT estimates for uncertain tasks.
✓ Answer checked by Claude Opus 5.5 (high effort)
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