Application of the PERT Method
We previously reported that the implementation of the activity takes time and needs resources. The PERT team may face a problem of how to estimate the time of each activity, as they see that reliance on a single estimate time is a weak assumption and is incompatible with the conditions of uncertainty, even under the best circumstances. The external environmental factors surrounding the project may cause a deviation in the times of the activities planned.
Accordingly, the PERT method has been found to overcome the time uncertainty. By estimating the time of each activity with three estimated time values, they will be gathered together with to reach a statistical probability to estimate time of the project completion. This method will apply as follows.
Optimistic time estimates the minimum possible time in which to do the activity, considering that all the factors affecting this activity are going smoothly. This time is usually low because all the circumstances must be good at the same time.
Pessimistic time estimates the maximum possible time in which to implement the activity. So it considers the worst cases happen to delay this activity. This usually provides a higher time period with low probability.
The "most likely time" presents the time period in which to perform this activity under normal conditions for all the factors affecting this activity. This value has the most probability of occurring.
The PERT method correlates the relation between the above three times by using a beta distribution as in Figure (2.11). The optimistic and pessimistic value precincts the limits of the distribution, but the most likely value present is the frequencies of the event occurring.
To define the activity time period, the activity mean time needs to be calculated, and this is done using the below equation. The summation of all the time periods on the critical path presents the mean time period for the project.