CPM &PERT

According to Schwalbe (2010), “Critical Path Method (Critical Path Analysis) is a network diagramming technique used to predict total project duration”.

The basic purpose of creating a PERT chart is to determine how much time will take to finish the whole project and can identify the earliest possible dates on which we can expect an activity to start or end (Kerzner, 2006). In PERT, user can easily analyze dates with its network diagram.

Optimistic time (a) - Time an activity will take if everything goes as well as possible

Pessimistic time (b) - Time an activity would take assuming very unfavorable conditions

Most Likely time (m) - Most realistic time estimate to complete the activity

(Rehan, 2012)

According to Rehan (2012),

To find the expected activity time (t), the beta distribution weights the estimates as follows

t = (a + 4m + b)/6

To compute the dispersion or variance of activity completion time, we use the following formula

Variance = ( ( b – a ) / 6 )^2

A series of activities that determines the earliest time by which the project can be completed is called the Critical Path of a project. (Schwalbe, 2010)

The differences between PERT and CPM

•  PERT uses three time estimates (optimistic, most likely and pessimistic) to derive an expected time. CPM uses one time estimate that represents the normal time.

•   PERT is probabilistic in nature, based on a beta distribution for each activity time and a normal distribution for expected time duration. This allows us to calculate the “risk” in completing a project. CPM is based on a single time estimate and is deterministic in nature.

•  Both PERT and CPM permit the use of dummy activities in order to develop the logic.

•  PERT is used for R&D projects where the risks in calculating time durations have a high variability. CPM is used for construction projects that are resource dependent and based on accurate time estimates.

(Kerzner, 2006)