PMBOK standard deviation formula is most often wrong!

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Lies, damned lies, and statistics” 

British Prime Minister Benjamin Disraeli

You might well ask what is “Standard Deviation.” Sounds like a regular deviant? And what could it possibly have to do with managing our projects, given we’ve all no doubt managed projects successfully without any knowledge of this peculiar statistical thing involving probability theory.

A statistics buff would tell us that Standard Deviation ia about variation from the Mean. In the project context Mean or Best Estimated Time (BET) is a weighted average that is best calculated using the Program Evaluation and Review Technique (PERT) formula. A low Standard Deviation indicates that pessimistic and optimistic durations are very close to the Mean – see the blue curve in the diagram below. A high Standard Deviation indicates that these estimates of duration are spread out over a large range – see the red curve. Standard Deviation is usually represented by … Read More »



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