Difference Between PERT & CPM

  Reviewed by: Michelle Seidel, B.Sc., LL.B., MBA
  Written by: Anam Ahmed      Updated November 08, 2018
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When working on complex multi-phase projects with multiple stakeholders, many organizations use established project management methodology to help them to meet their goals on time and on budget. The Program Evaluation and Review Technique (PERT) or Critical Path Method (CPM) are two proven project management techniques. Both have their strengths and weaknesses depending on the kind of project they are being used to manage.

Sometimes, PERT and CPM are used in tandem to manage projects with several relationships and dependencies between activities. PERT is typically used for research and development projects, whereas CPM is commonly used for construction-based projects.

Factoring in the Knowns and Unknowns

One of the key differences between PERT and CPM is how they treat the knowns and unknowns of a project. PERT is typically used to manage the uncertain activities within a project, which is why it often works well in research-based environments with uncertain outcomes. CPM, on the other hand, is used to oversee the well-defined activities of a project.

PERT works best for jobs or activities that are non-repetitive in nature, whereas CPM is the opposite. It is typically best for repetitive jobs where the outcome or result is predictable or known.

Taking into Account Time and Cost

Time and cost are two of the most important factors organizations consider when working on large-scale projects. CPM and PERT both treat time and cost differently. In PERT, time is the major factor that is controlled and monitored. Cost is not considered. Using PERT, organizations can pinpoint three precise time estimates for activities within the project. Those estimates take into account possible delays that can occur. Because PERT deals with unpredictable activities, it is imperative to have multiple time estimates for the project. To calculate the expected time to complete the project, the most likely scenario is multiplied by four, adding the most optimistic and pessimistic times and then dividing the result by six.

CPM is a method that weighs both time and cost and evaluates the time-cost trade-off for activities within a project. Unlike PERT, CPM provides only a one-time estimate, which is possible because CPM is for repetitive activities with known outcomes. Instead of a high-precision time estimate as in PERT, CPM time estimates are fairly reasonable. Critical and non-critical activities are differentiated in CPM, but not in PERT. To calculate the critical path using CPM, all of the required tasks are listed with a time assigned to each task and dependencies between the activities. The longest path from beginning to end is calculated with the earliest each activity can start and the latest it can finish without causing delay.

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Choosing Between PERT and CPM for Your Project

Deciding which method to use for your project will depend on the types of activities you will need to manage. It can be difficult to decide between PERT and CPM, but it will come down to whether your organization is dealing with knowns or unknowns, and considering both time and cost or just time.

Each technique helps organizations to manage risks, mitigate delays and ensure efficient completion of projects.

About the Author

Anam Ahmed is a Toronto-based writer and editor with over a decade of experience helping small businesses and entrepreneurs reach new heights. She has experience ghostwriting and editing business books, especially those in the "For Dummies" series, in addition to writing and editing web content for the brand. Anam works as a marketing strategist and copywriter, collaborating with everyone from Fortune 500 companies to start-ups, lifestyle bloggers to professional athletes. As a small business owner herself, she is well-versed in what it takes to run and market a small business. Anam earned an M.A. from the University of Toronto and a B.A.H. from Queen's University. Learn more at www.anamahmed.ca.

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