# Difference Between CPM and 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)

The following paragraphs provide a brief overview of the Program Evaluation and Review Technique (PERT).

### What is PERT?

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.

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.

### PERT Network Diagram

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 Estimates

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 Application & Usage

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.

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)

The following paragraphs provides basic details of Critical Path Method (CPM) schedule analysis technique.

### What is CPM?

Critical Path Method (CPM) is also a schedule development and analysis technique.

Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand developed Critical Path Method (CPM) in 1958.

### Diagramming Techniques in CPM

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.

Also read: Critical Path Method PMP Exam Questions

### CPM Application

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 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

AttributesCritical Path Method (CPM)Program Evaluation and Review Technique (PERT)
Year Introduced19581957
Authors / OrganizationMorgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand United States Navy, Booz Allen Hamilton and Lockheed Cooperation
ApplicationActivity duration are fairly known.Highly uncertain activity duration.
Project TypesConstructionResearch & Development
Estimate TypesDeterministicProbabilistic
Calculation MethodSingle point estimates3 point estimates
Network Diagram Method / TechniquePrecedence Diagramming Method (PDM)Arrow Diagramming Method (ADM)
Network Diagram TypeActivity-on-Node (AON)Activity-on-Arrow (AOA)
ControlsSchedule & CostSchedule
ComplexityLowHigh
PopularityHighLow
Schedule Risk CalculationAdditional techniques are requiredYes
Probability of Project CompletionAdditional techniques are requiredYes
Accuracy of estimatesLowHigh
ExplanationCritical Path Method Schedule AnalysisProgram Evaluation Review Technique PERT
FormulasCritical Path Analysis FormulasPERT Formulas

Also read: Critical Path Analysis Solved Example

## 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 is sufficient. However, PERT is an equally important scheduling technique necessary to broaden the project scheduling knowledge base.

## 9 thoughts on “Difference Between CPM and PERT”

1. rf says:

Hi,
Various other sites mention that the CPM method is used on Activity on Arrow diagrams. Please verify.
Thanks

1. Hi, RF, Establishing critical path for a schedule network diagram is not the same as Critical Path Methodology. Both PERT and CPM help to establsih critcal path of a schedule network diagram. However, for CPM technique Activity-On-Node diagraming technique is used but for PERT we need to use Activity-on-Arrow.

2. Robin M Foster says:

Wow, this was amazing. What a nice presentation. I’ve learned so much from you, already.

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