21 March, 2024

What’s the Difference between PERT and CPM?

By Mike Clayton

PERT and CPM are two forms of Network Chart that are represented differently, and take different approaches to calculating schedules. For both, the roots lie in the 1940s and the way the Manhattan Project was planned.

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Comparing PERT and CPM

My intention is not to give you a detailed explanation of either PERT or the CPM. Rather, to simply highlight the main differences between them.

Program Evaluation and Review Technique

  • 1950s
  • Originally developed by the US Navy and Booz Allen Hamilton
  • Activities on Lines – nodes connect tasks. The focus is on activities.
  • 3-point estimates for each duration – Most Likely, pessimistic (longest plausible) and optimistic (quickest plausible) estimates – not symmetric – weighted average: (O + 4L + P)/6.
  • Hence PERT is a probabilistic approach.
  • This makes PERT more suitable for projects that are more novel.
  • PERT was designed to optimize and control schedule only, by understanding the spread of possible schedule outcomes for each activity.
  • Popular in the R&D sector

Critical Path Method

  • 1950s (a little later than PERT)
  • Originally developed by Du Pont and Remington Rand
  • Activities on Nodes (boxes) with Arrows as linkages between them. The focus is on events.
  • Single-point estimates for activity durations are better when activities are more clearly defined.

    Hence CPM is a deterministic approach.
  • This makes CPM more suitable for projects that are more repetitive.
  • CPM is used to control schedule and budget, and helps calculate the trade-offs between them, using an understanding of float and critical or non-critical activities
  • Popular in the construction sector

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Mike Clayton

About the Author...

Dr Mike Clayton is one of the most successful and in-demand project management trainers in the UK. He is author of 14 best-selling books, including four about project management. He is also a prolific blogger and contributor to ProjectManager.com and Project, the journal of the Association for Project Management. Between 1990 and 2002, Mike was a successful project manager, leading large project teams and delivering complex projects. In 2016, Mike launched OnlinePMCourses.
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