PERT vs. CPM in Waterfall planning

In this post I discuss two foundational Waterfall project management approaches: Critical Path Management (CPM) and PERT, their differences, and why the CPM-method never should be used if a team lacks previous experience from similar projects.

“Critical Task Analysis’ is a method of project management which has its roots in the mid-50s and which was invented to cope with delays in plant turnarounds, to help to solve critical delays, and to determine how to “best reduce the time required to perform routine and repetitive tasks that are needed to support an organization” [Stelth and Roy, 2009, p. 17].

Interesting to note is that the CPM-methodology was developed by two unrelated organizations; The US Navy’s Fleet Ballistic Missile Program’ who named their approach to it ‘Project Evaluation and Review Technique’ (PERT), and DuPont—who on their side named it ‘Critical Path Method’ (CPM) [Esposito, 2018].

Many project managers use the terms PERT and CPM interchangeably with both methods sharing the same general hypotheses which include:

• Projects can be broken down into identifiable and quantifiable tasks.
• Some tasks might depend on the completion of previous tasks—while others might be independent of the tasks ahead and can be undertaken at any given time.

[Lowe, 1971]

Key differences between CPM and PERT

The most notable difference between CMP and PERT is that in the former you only do one estimation for each identified task while in PERT you use three:

• P: The most pessimistic
• O: The most optimistic
• M: Most Likely

As such, PERT is the preferred method when time management is of primary importance; if there are many unknowns such as if an organization has little experience from similar projects; and if the project is “non-repetitive”. CPM—on the other hand—is the preferred method if a project is recurring in nature, when time estimations can be grounded in historical data, and/or if cost control is of primary focus [Stelth and Roy, 2009, p. 11; McNeil, Frey and Embrechts, 2015; Surbhi, 2018; Emelda, 2011].

Fig 1, below, illustrates some of the key differences between PERT and CPM:

So which method is the best?

As noted by Stelth and Roy [2009]; Surbhi [2018] and McNeil, Frey and Embrechts [2015], CPM and PERTH serve the same core purpose of reducing risk, identify critical tasks, and predicting project completion times. While the CPM technique is a deterministic tool which allows project planners control of cost and time, the method is primarily aimed at projects with predictable activities and tasks and thus, as noted by Stelth and Roy [2009, p. 18], also demand experience from similar projects and tasks to enable informed estimations. The PERT-method, on the other hand, is a probabilistic tool aimed at projects with unpredictable tasks and uses three time estimates compared to one in CPM to control activities and to estimate time.

While the PERT and CPM methods share some general hypotheses, it is important to note that they also have some significant differences and are created to solve fundamentally different problems. Consequently, any discussion about ‘Critical Path Analysis’ need to cover both PERT and CPM, and for project managers it is imperative to learn about both to enable educated decisions of which method is most suitable for the projects they currently work on.

References

Emelda, M. (2011). Difference Between CPM and PERT | Difference Between. [online] Differencebetween.net. Available at: http://www.differencebetween.net/business/management-business/difference-between-cpm-and-pert/ [Accessed 21 Feb. 2018].

Esposito, E. (2018). The Ultimate Guide to the Critical Path Method. [online] Smartsheet. Available at: https://www.smartsheet.com/critical-path-method [Accessed 21 Feb. 2018].

Haidar, A. (2016). Construction program management.

Hulet, D. (2015). Practical Schedule Risk Analysis. Princeton University Press, p.Chapter 2.

Lowe, C. (1971). Critical path analysis by bar chart. London: Business Books.

McNeil, A., Frey, R. and Embrechts, P. (2015). Quantitative Risk Management: Concepts, Techniques and Tools.

Mulcahy, R. (2013). PMP exam prep. [Minnetonka]: RMC Publications.

Stelth, P. and Roy, G. (2009). Projects’ Analysis through CPM (Critical Path Method). School of Doctoral Studies (European Union), [online] 1(1). Available at: http://www.iiuedu.eu/press/journals/sds/sds1_july_2008/05_SECC_01.pdf [Accessed 21 Feb. 2018].

Surbhi, S. (2018). Difference Between PERT and CPM (with Comparison Chart) – Key Differences. [online] Key Differences. Available at: https://keydifferences.com/difference-between-pert-and-cpm.html [Accessed 21 Feb. 2018].