Don't Forget Standard Deviation!

Andy Jordan is President of Roffensian Consulting S.A., a Roatan, Honduras-based management consulting firm with a comprehensive project management practice. Andy always appreciates feedback and discussion on the issues raised in his articles and can be reached at [email protected]. Andy's new book Risk Management for Project Driven Organizations is now available.

  New Practitioners   Risk Management   Schedule Management   Scheduling  

If you go through any kind of foundational project management training for traditional project delivery approaches, you’ll inevitably spend time learning about estimating. There are a number of tools and techniques, but one of the more common—and more reliable—is something called PERT, or Program Evaluation Review Technique. (It’s far from perfect, but that’s a topic for another day.)

Developing a PERT estimate requires consideration of three things:

1. The optimistic estimate (O): This is the best-case scenario, the minimum amount of time that a task could take if everything went well.
2. The most likely estimate (ML): This is the amount of time that represents the expected time needed in most cases.
3. The pessimistic estimate (P): This is the worst-case scenario, the realistic maximum amount of time that the task could take if everything were to go wrong.

The PERT estimate for the overall task is then calculated using the formula:

(O + 4xML + P) / 6

So, if the optimistic estimate for a task is 6 days, the pessimistic is 18 days and the most likely is 9 days, then we would calculate our estimate as:

(6 + 4x9 +18) / 6 = 10 days

For many, that’s where PERT stops, but there is another element that I believe can be of great value in understanding the uncertainty and risks associated with tasks that have been estimated using…

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