How to Know the Difference Between PERT and CPM

Discover the vital distinction between PERT and CPM. Learn how PERT employs probability for time estimates, while CPM relies on deterministic calculations. Master the art of efficient project scheduling and critical path identification. Find clarity amidst complexity.

PERT and CPM are two network-based, analytical techniques in project management to manage tasks efficiently.  They are also often used together when it comes to completing a project. But, the primary difference between PERT and CPM lies in how the time for tasks is calculated. Let’s find out how they are different from each other in more detail. 

Difference Between PERT and CPM

Here are some of the main points of difference between PERT and CPM. 

What is PERT?

PERT stands for Program Evaluation and Review Technique. It represents the time required to complete a project. A project manager creates a PERT chart for planning and scheduling tasks within a project. 

In a PERT chart, there are three different types of time estimates that are probabilistic 

1. Optimistic Time – It is the least amount of time for an activity to be completed
2. Pessimistic Time – It is the maximum amount of time required to complete the activity
3. Most Likely Time – It is a reasonable amount of time required for an activity provided there are no bottlenecks involved

It is also a type of Three Point Estimate technique as the formula is only probabilistic. According to Project Management Institute, for any given activity, the estimated time is calculated as

Estimated Time = (Optimistic + 4 x Most Likely + Pessimistic) /6 

Developed as a tool for managing US Navy projects back in the 1950s, PERT analysis is based on a network diagram. It contains arrows or arcs for activities and nodes (circles) for specifying events. The arrows in the chart define the sequence of activities and events. The path defines the start to finish in the network diagram, known as the path or in some circles - critical path. 

One of the main differences between PERT and CPM is that PERT is focused on the arrows and not the nodes like CPM. The PERT analysis is based on the activity-on-arrows, where each event/milestone is joined by the arrows.

The videos below from experts in the domain will help you further. 

What is CPM?

CPM or Critical Path Method is used for creating the timeline for the project. The critical path here is the longest path in a network diagram, which can also mean it is the shortest duration required for project completion. 

Some common terms in CPM are

• Early Start – The earliest time when the project can be started
• Late Start – It describes the latest time by when the project should start without affecting the critical path
• Early Finish – It is the earliest time by when the project is completed
• Late Finish – It is the latest time by when the project should be completed without getting delayed

As you saw earlier that PERT is focused on activity-on-arrows, CPM is based on activity-on-nodes. It does not link the milestones like PERT, but only links from one event to the next. 

CPM was developed in the 1950s by James E. Kelley of Remington Rand and Morgan R. Walker of DuPont. Unlike PERT, the focus was on reducing costs and delivering projects on time. 

Another way to distinguish between PERT and CPM is that CPM is not a probabilistic model like PERT. CPM is more deterministic which looks into independent and interdependent activities, based on which you can analyse whether the time and cost required for the completion can be reduced. 

Real-Life Example of Difference Between PERT and CPM

Let’s consider a tech company developing a new software product. The project involves tasks like concept development, coding, testing, documentation, and product launch. Here’s how PERT and CPM can be applied:

Using PERT, the project team estimates the time required for each task in the software development process, taking into account the uncertainties involved. For example:

Coding: Optimistic Time = 20 days, Pessimistic Time = 40 days, Most Likely Time = 30 days.

Testing: Optimistic Time = 15 days, Pessimistic Time = 25 days, Most Likely Time = 20 days.

The PERT analysis helps in understanding the probable time duration for each task and identifies the critical path for the project.

There’s no need for such weighted averaging in CPM, as it uses the most likely duration directly as the expected time. CPM doesn’t explicitly use probability distributions. It focuses on deterministic task durations to calculate the critical path.

Why PERT and CPM are Used Together Today 

While both terms were developed independently in the late 50s, they are almost used in software development packages together. In some contexts, they are used interchangeably. For instance, project management software such as Microsoft Project help users use both these techniques together. Historically, PERT was used when uncertainty in activity durations was high and you needed to estimate variability in project completion times. While, 
CPM helped when project deadlines were tight and you needed to determine most cost-effective crashing options. In the current context, to deal with duration uncertainty and investigate time-cost tradeoffs, you need both together.   

Parting Thoughts

So these are the basic differences between PERT and CPM. Nowadays, organisations use both together, and the distinctions between the two are not much when it comes to large projects. 

Consider taking project management courses to theoretically and practically know the varying bases of CPM PERT difference. 

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