PERT assumes a beta probability distribution (beta分布) for the time estimates. For a beta distribution, the expected time for each activity can be approximated using the following weighted average:
Expected time (期望) = ( Optimistic + 4 x Most likely + Pessimistic ) / 6
This expected time may be displayed on the network diagram.
To calculate the variance for each activity completion time, if three standard deviation times were selected for the optimistic and pessimistic times, then there are six standard deviations between them, so the variance is given by:
[ ( Pessimistic - Optimistic ) / 6 ]2