PERT in Construction: Scheduling for Uncertainty

Contractors with an array of planning tools and techniques are able to create more precise project timelines, customize preparation for each job and improve communication with owners and subcontractors. The Program Evaluation and Review Technique (PERT) is a method that, while not widely used in construction, can be effective at improving planning for a variety of tasks, milestones and projects. 

PERT is commonly used to refer to two different but not mutually exclusive tools: 

• A formula that can help establish probabilistic time estimates for tasks or projects, particularly those with a high degree of uncertainty
• A chart that can be used to represent probabilistic durations between project milestones

This article explores PERT, including when to use it, examples, and how it can complement and improve other planning tools and techniques.

PERT Formula for Estimating Task Duration

PERT is a tool to help estimate task durations. Experienced contractors are usually able to accurately estimate the duration of most activities, either because they’ve done the task before or because they can reference when someone else did something similar. 

However, the duration of some activities can still be difficult to forecast, especially when they’re part of a project that’s extremely unique or has a lot of uncertainty. 

PERT uses a simple equation to calculate a probabilistic time estimate by creating a weighted average of three different time estimates:

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

Those times are plugged into the following formula, with additional weight given to the most likely time:

Here's an example: An old factory is being renovated to become an office building. Figuring out how long to plan for installing drywall is proving to be difficult, given the unique features of the building and layout of the space. The following time estimates for installing drywall are established: 

• Optimistic Time: 12 days
• Most Likely Time: 15 days
• Pessimistic time: 20 days

According to the formula, the estimated duration is 15.3 days:

[12 + (4*15) + 20] ÷ 6 = 15.3

The estimate of 15.3 days is similar to the most likely time of 15 days, but is helpful because it suggests that an additional day should be allocated to complete the task.

How Construction PERT Charts Work

PERT charts are a visualization technique for showing the estimated duration between major project milestones. The chart sequences project milestones, connects them with arrows in the order they need to be done and shows the duration between two milestones. This type of chart is known as an activity-on-arrow.

The PERT chart shows milestones, which sets it apart from charts used with other planning techniques, such as the one used with the Critical Path Method (CPM), that look similar but are used to define and sequence tasks and activities in great detail. 

When the duration between the milestones is unknown or uncertain, the PERT equation can be used. However, the chart is also useful when durations are known.

PERT charts can share project timelines with owners without giving excessively — and potentially confusing — detailed information. Additionally, milestones are often when payments are submitted or received, so the PERT chart can be used to help plan budgets and share information with subcontractors. 

Example of a PERT Chart

A contractor wants to share the sequence of milestones with the project’s owner. They create a PERT chart where the first milestones are “Mobilize to Site” and “Excavation.” 

Because the duration of time between these two milestones is uncertain, the contractor uses the PERT equation to establish an estimate, shown above the arrow. This process could be continued for each milestone.

Using PERT with Other Scheduling Methods

PERT can be used to complement or improve other planning methods. When using other tools and techniques, the PERT equation can be used to estimate the duration for tasks that are unknown or uncertain. 

The full chart from the PERT chart example above uses red to highlight the critical path — which is the longest sequence of tasks that can’t be delayed without having to change the schedule — and is often found through the critical path method (CPM). The PERT chart, as opposed to the much more detailed chart used in CPM, can be a more effective big-picture tool to share with the owner.

Many contractors gain enough experience that accurately estimating the duration for most activities isn’t a challenge. However, PERT allows them to remain nimble and precise when faced with new or unknown tasks or projects.

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