Activity variance measures uncertainty in an activity’s estimated duration. Using optimistic time a and pessimistic time b, calculate variance as ((b − a) / 6)². Higher values indicate less predictable durations; lower values indicate more consistent estimates. section 13.4
For a project, sum variances only for activities on the critical path, assuming activity times are statistically independent. The square root gives project standard deviation, which supports completion-probability analysis with a normal distribution and z-score. section 13.4
Automated reporting tool
Activity A has optimistic and pessimistic estimates of 2 and 4 weeks, producing variance 1/9. A Python calculation using SciPy combines project mean, standard deviation, and due date to estimate completion probability. section 13.4
