A controversy has developed over the years between the proponents of PERT and CPM. The commonly debated questions are: Which of these methods originated first? Which is the better one? Are both of them practically the same? To understand the various distinguishing features of the two and their spheres of application, let us first discuss their original differences and then the subsequent developments. All controversial issues, which are only of an academic interest with no practical significance have been avoided.


Original Differences

CPM was developed for planning, scheduling and control of civil works, while PERT originated in response to the complexities of the uncertainity in research and development projects for controlling their multifarious time schedules. Originally, thus, their fields of application were quite different.

In network modelling, CPM laid emphasis on breaking the projects into various works or activities. In PERT, the project breakdown was in terms of milestones which were planned to occur during its execution. Therefore, CPM was activity-oriented whereas PERT was event-oriented.

Originally, the application of CPM was confined to construction works where the activities were familiar and their duration could be easily estimated from the one-time estimate. Since PERT was designed to cope with uncertainties, it used the three-time estimate.

In CPM, activity durations where related to costs. This provided a means of assessment of different activity durations with varying costs and made crashing of activities possible. PERT dealt with events and their probable time of occurrence. This enabled adoption of probabilistic approach in time scheduling.

The CPM schedule enabled optimization of resources as the activity durations were defined in terms of resources employed. This was not possible with PERT.

CPM, which used the one-time estimate, was simpler to follow, while PERT required a statistician to interpret the results.


Later developments

As the various drawbacks of PERT gradually became apparent, further studies were initiated by the US Defence Department. Notable among these were the PERT/TIME and PERT/COST. In the later, the concept of single-time estimate of activities could be equally employed.

In the CPM field, the concept of three-time estimate was introduced into networks which were primarily designed for controlling the time factor rather than the resources. In recent years, further developments in PERT and CPM have made CPM appear more like PERT, and the subject of network analysis has come to be known as PERT/CPM. To distinguish these two main network techniques, the event-oriented networks using three-time estimate for activity durations having uncertainties can be termed as a PERT network, while the other which is activities-oriented and uses one-time estimate may be referred to as the CPM.