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Project Management & Agile · Advanced

Standard Deviation (PERT)

Measure of estimate uncertainty calculated as (Pessimistic - Optimistic) / 6.

Definition

Variance is the square of the standard deviation; path variances add, so path standard deviation is the square root of the sum of activity variances. Ranges of plus or minus 1, 2, and 3 sigma cover about 68%, 95%, and 99.7%.

Example

P=14, O=2: SD = 2; 95% range for an estimate of 6 is 2 to 10.

Exam tip

Add variances, not standard deviations, across a path.

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