Program Evaluation and Review Technique (PERT) and Critical Path Method both are project schedule development and analysis techniques. Both these techniques establish the critical path in a schedule network diagram. However, both CPM and PERT use different approach to arrive at the critical path and activity duration estimates. The following blog post highlights difference Between CPM and PERT.
Program Evaluation and Review Technique (PERT)
In 1957, United States Navy, Booz Allen Hamilton and Lockheed Cooperation developed Program Evaluation And Review Technique (PERT) to cater for Polaris missile / submarine projects.
Program Evaluation And Review Technique (PERT) is a schedule development and analysis technique. It represents the logical sequence of project schedule activities with their relationship in a graphical format.
However, PERT uses Arrow Diagram Method (ADM) technique to represent the logical sequence of project schedule activities. Activity-on-Arrow (AOA) diagramming method is an example of ADM technique to construct the project schedule network logic.
PERT finds its usage when there is a high degree of uncertainty. Moreover, it uses weighted average formula of three-point estimation technique to calculate the expected activity duration. This helps to calculate activity duration when there is a high degree of uncertainty. Since PERT accounts for uncertainty and risk the expected duration is more accurate than the single point estimation technique.
Moreover, PERT uses probabilistic time estimates to determine the probability of project completion date. Further, it also facilitates calculation of risk involved in project completion date based on beta distribution of each activity time and normal distribution of the expected time.
PERT further allows representation of large amounts of data for further analysis and modelling. Effect of changes on schedule as a result of changes in resources can be evaluated fairly easily.
Program Evaluation and Review Technique mainly helps to control time element of projects. PERT is also used for those projects where percent complete is impossible to determine except for completed milestone.
Research and Development projects especially the development side of the project uses PERT.
Critical Path Method (CPM)
Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand developed Critical Path Method (CPM) in 1958.
Critical Path Method (CPM) is also a schedule development and analysis technique. The critical path method analysis uses Precedence Diagram Method (PDM) technique to construct the schedule network logic.
Precedence Diagramming Method is a technique that uses logical relationship among schedule activities to construct the network diagram. Activity-On-Node (AON) is one such technique that uses PDM method to construct the schedule network logic.
Moreover, Critical Path Method (CPM) finds application in construction projects. Further, Critical Path Method uses deterministic (single point) activity time estimates. However, CPM can control both time and cost aspect of the project. Moreover, the modern-day scheduling software also provide PERT analysis of the schedule developed.
Unlike PERT, critical path method controls both time and cost aspect of a project.
Today, Critical Path Method (CPM) is the most popular schedule network analysis technique. Moreover, most of the planning and scheduling programs use CPM technique to analyse the schedule. Hence, out of the two, Critical Path Method is the most widely used project schedule network analysis technique.
CPM and PERT Comparison Chart
|Attributes||Critical Path Method (CPM)||Program Evaluation and Review Technique (PERT)|
|Authors / Organization||Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand||United States Navy, Booz Allen Hamilton and Lockheed Cooperation|
|Application||Activity duration are fairly known.||Highly uncertain activity duration.|
|Project Types||Construction||Research & Development|
|Calculation Method||Single point estimates||3 point estimates|
|Network Diagram Method / Technique||Precedence Diagramming Method (PDM)||Arrow Diagramming Method (ADM)|
|Network Diagram Type||Activity-on-Node (AON)||Activity-on-Arrow (AOA)|
|Controls||Schedule & Cost||Schedule|
|Schedule Risk Calculation||Additional techniques are required||Yes|
|Probability of Project Completion||Additional techniques are required||Yes|
|Accuracy of estimates||Low||High|
|Formula||PMP Formulas||Project Management Formulas|
CPM and PERT Conclusion
Project Management Body of Knowledge (PMBOK)® recognizes only CPM and other techniques like Critical Chain Method (CCM) as a scheduling methodology. Moreover, PMBOK mentions PERT’s three point estimation technique as a tool for duration and cost estimates. As a matter of fact, from PMP certification exam point of view understanding critical path analysis is sufficient. However, PERT is an equally important scheduling technique necessary to broaden the project scheduling knowledge base.
Also read: Critical Path Analysis Solved Example