Three time estimates in PERT and their relationship with expected time and its variance in the project

Three time estimates in PERT and their relationship with expected time and its variance in the project. (Apr 2003) (Oct 2005) (Apr 2006)

In PERT, there are three time estimates –

1. Optimistic time (shortest possible time ) Denoted by “a” or “to”
2. Most likely time (middle value) Denoted by “m” or “tm”
3. Pessimistic time (longest possible time) Denoted by “b” or “tp”

From these three time estimates, we can calculate Expected time of an activity, Standard deviation of activity and Variance of activity.

1. Expected time of the activity = (a + 4m + b ) / 6
2. Standard deviation of the activity = (b – a) / 6
3. Variance of an activity = Square of standard deviation
4. Variance of critical path = Total of variances of all critical activities