PERT vs. CPM: Comparing Construction Scheduling Methods

Contractors should have an array of planning techniques to develop reliable schedules, accurate estimates and effective contingencies for even the most complex or uncertain projects. The Critical Path Method (CPM) and the Program Evaluation and Review Technique (PERT) are two helpful approaches to scheduling. 

While CPM is much more common than PERT in construction, both scheduling approaches can serve different purposes. PERT gives a high-level overview of project milestones, while CPM provides a more detailed look at duration estimates for each activity.

In fact, these methods are not necessarily mutually exclusive. When approaching a task or project with an irregular or unpredictable scope, schedulers can effectively use both methods together to build a more accurate, achievable timeline even in the face of uncertainty. 

Overview of PERT

PERT is a technique in which probabilistic time estimates are used to accurately forecast the duration of an activity or project, particularly those with a high degree of uncertainty. PERT uses a simple equation to calculate a weighted average of different time estimates. 

The PERT method requires you to produce three time estimates: 

• Optimistic Time (OT)
• Most Likely Time (MT)
• Pessimistic Time (PT) 

You can then use the formula to calculate an average of the three, giving additional weight to the most likely time. The formula multiplies 

PERT Weighted Estimate = OT + (4 * MT) + PT ÷ 6

When to Use PERT

PERT can be helpful for any task or activity where the duration isn’t known or is uncertain. For example, PERT would be useful in planning for constructing a highly specialized factory that is unlike other projects.  

The technique is event-oriented, meaning it focuses on milestones and timelines and can be used for whole projects or for individual tasks. PERT can help build schedules that are less likely to have delays, which is why it is often deployed during the early stages of planning a project.

PERT Visualization

PERT is most often visualized as a sequence of project milestones, with the duration between them shown on the line connecting each one, known as activity-on-arrow.

PERT is most often visualized as a sequence of project milestones, with the duration between them shown on the line connecting each one, known as activity-on-arrow.

For example, two initial milestones on a project (mobilize to site and excavation) may be shown as nodes on a sequence of tasks. The time estimated between mobilization and excavation is shown on the arrow between the milestones. 

Overview of CPM

The Critical Path Method (CPM) is an approach to scheduling that orders the tasks to complete a project (or phase of a project) and identifies the sequence of tasks that are most critical for the project to be delivered on time. CPM is one of the most common planning techniques in construction.

CPM defines a task’s latest and earliest start and finish dates, which can then be used to create actionable schedules for tasks found in the work breakdown structure (WBS), a common organizational system for complex undertakings like construction projects.

Identifying the Critical Path

The critical path is the longest sequence of tasks in a project or segment. Schedulers use the critical path method to identify the amount of float or slack available in each activity. The critical path is identified as the sequence of tasks without any float. 

While all tasks are necessary to finish a project, the critical path consists of activities that can’t be delayed without having to change the schedule of activities. Subsequent activities depend on the completion of critical tasks. 

Non-critical tasks, by contrast, are activities with more flexibility or float: They can often be completed concurrently with other activities. By identifying this critical sequence, contractors can prioritize allocation of resources — including equipment, materials and labor — while also determining when to schedule non-critical activities.

There are many ways to create a schedule using the Critical Path Method, including many digital planning tools that make it easy to organize, visualize, and share a schedule. Often, the planning tools include a Gantt chart to visualize a task’s duration, potential flex time, and overlapping work periods with different tasks.

When to Use CPM

CPM is a sequencing technique, meaning it focuses on identifying necessary tasks and how they relate to other tasks. In identifying critical and non-critical tasks, CPM is most useful in allocating resources. 

However, CPM doesn’t help determine how long a task will take, so it’s often used for planning projects where most of the tasks and activities are familiar, and their durations are easy to anticipate. Since construction companies often have historical data on the duration of common activities – even if they are only stored in the scheduler’s memory – they can frequently estimate timelines without using a PERT formula. 

CPM Visualization

Like PERT, CPM is typically visualized in a sequence of activities. However, each activity and information about its duration is shown on the node, known as activity-on-node. 

Though the details can vary depending on the scheduler’s preference, a CPM schedule will generally display the following details for each activity:

• Activity name
• Duration 
• Earliest start time
• Earliest finish time
• Latest start time
• Latest finish time
• Float or slack

The project manager will follow a series of steps using each of these details to identify the float, which will determine the critical path. This is why it is known as the Critical Path Method.

CPM vs PERT

CPM and PERT are not mutually exclusive scheduling methods. In fact, construction schedulers can use them both. However, CPM is generally much more common in construction. 

Using PERT & CPM Together

The difference between PERT and CPM can be a bit confusing, because the terms can be used in different contexts – they can refer either to the type of calculation or the type of visualization. 

For example, PERT can refer either to the formula (shared earlier in the article) or the milestone chart. The PERT formula can be used in the development of a CPM schedule since it is simply a calculation to estimate a task’s duration. Similarly, a PERT schedule will almost always show the critical path, even when the Critical Path Method wasn’t used to identify it. 

As contractors create a schedule using CPM, they will often be more confident in some of their duration estimates than others. With tasks of uncertain duration – or when historical data isn’t available to provide a more reliable estimate – they may use the PERT formula to calculate a duration. 

Different schedule visualizations may also be used for different project stakeholders. For example, a project manager can prepare a PERT schedule for an owner who wants to see project milestones without the distraction of extra details. However, when discussing the schedule with the superintendent, they would generally require the level of insight provided by a CPM schedule. 

A good planner should have many tools that can be deployed, and CPM and PERT can be used separately or together to help create schedules that are precise and adaptable to the specific needs of each project.

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