How to Calculate Probability in PERT

by L.P. Klages ; Updated September 26, 2017
A project manager uses a PERT chart.

A Project Evaluation and Review Technique (PERT) is a tool that project management uses to manage a project. The PERT chart contains information about subprojects and estimated completion times. Each subproject is assigned an estimated completion time, based upon probability distributions for the expected completion time for the subproject from start to finish. The probability distribution for expected time is calculated by the following equation: Expected time = (Optimistic Time + 4 x Most likely Time + Pessimistic Time) / 6.

Add the Optimistic Time for a subproject to the Pessimistic time for the same subproject. For example, if your subproject’s optimistic time is one day, and Pessimistic Time is seven days, the total for this step is eight days.

Multiply the Most Likely Time by four. If the most likely time is two days, then two days times four is eight days.

Add your answer from Step one to your answer from Step two. For the example, eight days + eight days is 16 days.

Divide your answer in Step 3 by 6. For the example, 16 days / 6 is 2.67 days (rounded). 2.67 days is the expected time for this subproject.

Repeat Steps 1 through 4 for each subproject in your PERT Chart. When you finish, you'll have calculated expected values for each step of your entire project schedule.

Tips

  • When in doubt, overestimate the Pessimistic Time for a subproject. It's always better to be early, rather than late, so giving yourself plenty of time for each subproject minimizes the impact of any delays.

About the Author

L.P. Klages is an entrepreneur and software developer, concentrating on information theory, software user experience, and mathematical modeling. He has been writing about technology and the business of technology since 1999. His articles have appeared on many sites, including GameDev.net, KenSharpe.net, and eHow. Klages attended Jacksonville University in Jacksonville, Fla.

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