Project variance measures uncertainty in the planned completion time by summing the variances of activities on the critical path, assuming statistically independent activity times. For an activity with optimistic estimate a and pessimistic estimate b, calculate variance as ((b − a) / 6)². A larger value indicates less predictable duration. section 13.4
Use project variance to convert schedule uncertainty into a completion-time probability:
- Sum only the variances of critical-path activities. section 13.4
- Take the square root of project variance to obtain the project standard deviation. section 13.4
- Compute z = (due date − mean project duration) / standard deviation, then evaluate the normal cumulative probability. section 13.4
