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Difference between CPM and PERT

Short answer

CPM uses deterministic time estimates for repetitive projects, while PERT uses three probabilistic estimates for uncertain R&D projects. Critical path is the longest path determining project duration. Float is delay allowed without delaying project; event slack is difference between latest and earliest event times in PERT.

Differentiate between CPM and PERT. Explain the terms critical path, float and event slack.

Both CPM (Critical Path Method, DuPont 1957) and PERT (Program Evaluation and Review Technique, US Navy, Polaris project 1958) are network techniques for planning, scheduling and controlling projects.

BasisCPMPERT
OriginIndustrial/construction projectsResearch and development projects
Time estimateOne deterministic timeThree estimates: a, m, b
NatureDeterministicProbabilistic
OrientationActivity orientedEvent oriented
CostTime-cost trade-off (crashing) is consideredMainly time; cost not considered
UseRepetitive, well-known jobsNon-repetitive, uncertain jobs
ProbabilityNot computedProbability of meeting a date is found
DistributionNot usedBeta distribution

Terms

  • Critical path: the longest path through the network; it fixes the shortest possible project duration. Any delay in a critical activity delays the whole project.
  • Total float: the time by which an activity can be delayed without delaying the project completion: TF=LS−ES=LF−EFTF = LS - ES = LF - EF. Critical activities have zero float.
  • Free float: delay possible without affecting the earliest start of the next activity.
  • Event slack (PERT): for an event, slack=TL−TE\text{slack} = T_L - T_E, the difference between latest allowable and earliest expected event times. Events on the critical path have zero slack.

✓ Answer checked by Claude Opus 5.5 (high effort)

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