PERT was originally developed for the Polaris Fleet Ballistic Missile Program of the US Navy in between 1958 to 1959. There is a specific difference between PERT and CPM (Critical Path Method). If someone considers CPM, then it is activity oriented and primarily focused on time along with cost. Considering the PERT, it is event oriented and primarily focused on time only.

While calculating the PERT, probability distribution is specified by most likely estimates of activities that include:

1. Duration (t111)
2. Optimistic Duration (to)
3. Pessimistic Duration (tp)

As it is event oriented, beta distribution will be considered for the calculation, hence-

Mean = to + 4tm + tp/ 6

Variance = (tp – to/6)2

The network construction in PERT is similar to CPM, thus, the critical path is identical to CPM. Here, the earliest and latest event times will be random variables.

Once the mean and variance are calculated, the critical path can be determined. Hence, probability statements can be calculated and it will be considered for the total event duration with slack at the event. Moreover, the critical path will be based on expected time and probability will show the duration of completion of the project.  