Program Evaluation Review Technique (PERT) Chart Explained

PERT Chart

Zoe Hansen / Investopedia

What Is a Program Evaluation Review Technique (PERT) Chart?

A program evaluation review technique (PERT) chart is a graphical representation of a project's timeline that displays all of the individual tasks necessary to complete the project. 

As a project management tool, the PERT chart is often preferred to the Gantt chart because it identifies task dependencies. However, a PERT chart can be more difficult to interpret.

Key Takeaways

  • PERT charts were first created by the U.S. Navy's Special Projects Office in 1950s to guide the Polaris nuclear submarine project.
  • A PERT chart uses circles or rectangles called nodes to represent project events or milestones. These nodes are linked by vectors, or lines, that represent various tasks and their dependencies.
  • A PERT chart allows managers to evaluate the time and resources necessary to manage a project.

How Does a PERT Chart Work?

A project manager creates a PERT chart in order to analyze all of a project's tasks while estimating the amount of time required to complete each one. Using this information, the project manager can estimate the minimum amount of time required to complete the entire project.

This information also helps the manager develop a project budget and determine the resources needed to accomplish the project.

A PERT chart uses circles or rectangles, called nodes, to represent project events or milestones. The nodes are linked by vectors or lines that represent various tasks.

Dependent tasks are items that must be performed in a specific manner. For example, if an arrow is drawn from Task No. 1 to Task No. 2 on a PERT chart, Task No. 1 must be completed before work on Task No. 2 begins.

Items at the same stage of production but on different task lines within a project are referred to as parallel tasks. They're independent of each other, and occur at the same time.

A well-constructed PERT chart looks like this:

Image
Image by Sabrina Jiang © Investopedia 2020 

How Is a PERT Chart Interpreted?

A PERT chart is a visual representation of a series of events that must occur within the scope of a project’s lifetime. The direction of arrows indicates the flow and sequence of events required.

Dotted activity lines represent dummy activities—items that are located on another PERT path. Numbers and time allotments are assigned and shown inside each vector.

These charts have their distinct definitions and terms, the most important of which anticipate how long it will take to finalize a project. "Optimistic time" refers to the shortest duration. "Pessimistic time" is the longest it might take. The "most likely time" indicates a reasonable estimate of the best-case scenario, whereas "expected time" accounts for problems and obstacles.

Advantages and Disadvantages of PERT Charts

A PERT chart is similar to a critical path analysis (CPA), another method that project managers use to chart tasks in a project. The main difference between the two is that a PERT chart uses various time frames and probability terms when estimating each project stage.

As a project management tool, PERT charts have distinct advantages and disadvantages.

Advantages

A PERT chart allows a manager to evaluate the time and resources necessary to complete a project. It also allows the manager to track required assets during any stage of production in the course of the project.

PERT analysis incorporates data and information supplied by a number of departments. This combining of information encourages department responsibility and identifies all responsible parties across the organization.

It also improves communication during the project and it allows an organization to commit to projects that are relevant to its strategic positioning.

Finally, PERT charts are useful input for what-if analyses. Understanding the possibilities concerning the flow of project resources and milestones allows management to achieve the most efficient and useful project path.

Disadvantages

The information that goes into a PERT chart can be highly subjective. They may include unreliable data or unreasonable estimates for cost or time.

PERT charts are deadline-focused and might not fully communicate the financial positioning of a project.

Creating a PERT chart is labor-intensive, and maintaining and updating the information requires additional time and resources. Continual review of the information provided, as well as the prospective positioning of the project, is required for a PERT chart to be of value.

Example of PERT Charts

PERT charts were first created by the U.S. Navy's Special Projects Office in 1950s to guide the Polaris Fleet Ballistic Missile project. Using the PERT model, Navy project managers were able to evaluate the progress of their project, determine the validity of their schedules, and estimate the effects of proposed changes to the established plans.

The success of this initial use of PERT charts has caused them to be used ever since in many industries.

PERT Chart vs. Gantt Chart

The Gantt chart goes back even further than the PERT chart. American mechanical engineer Henry Gantt designed it sometime around 1910 to 1915 and it remains the most widely used project management chart.

Like the PERT chart, the Gantt chart is a graphical depiction of a project timeline. Presented as a horizontal bar chart, it records the start and completion dates of each element of a project and indicates their interdependencies.

The two charts are used in similar ways. Both are used to define all of the key tasks that must occur to get a project completed. Both are useful in determining the time, money, and resources needed. And both can be used to monitor a project's progress to completion.

Key Differences

Early versions of the Gantt chart did not show dependencies. That is, it didn't indicate where a delay in completion of one task could hold up the start of another task, potentially throwing a whole project off schedule. Later versions of Gantt corrected this.

Some say that a PERT chart makes it easier to see and track a project's critical path. The critical path is the minimum time it will take to complete a project, based on the longest path from start to finish.

Some project managers prefer to use a PERT chart as part of the planning stage of a project and a Gantt chart to monitor its exercution.

What Are the Main Components of a PERT Chart?

A PERT chart displays all of the key deliverables necessary to complete a project.

It indicates the amount of time and resources needed to perform each task and the person or department responsible for each.

The key deliverables or tasks are displayed with arrows that indicate the order in which each must be completed, and the dependencies among them.

How Can I Create a PERT Chart?

Microsoft Excel can be used to create a PERT chart. Custom software with ready-made templates, such as Lucidchart, also are available.

Why Would I Use a PERT Chart Instead of a Gantt Chart?

A PERT chart requires a project manager to think through three possible timelines: "Optimistic time" is the shortest possible route to completion. "Pessimistic time" is the longest it might take if everything goes wrong. The "most likely time" is a reasonable estimate of the best-case scenario.

From the outset, the project manager has defined the ideal outcome of a project but also identified the possible barriers to achieving that outcome.

Article Sources
Investopedia requires writers to use primary sources to support their work. These include white papers, government data, original reporting, and interviews with industry experts. We also reference original research from other reputable publishers where appropriate. You can learn more about the standards we follow in producing accurate, unbiased content in our editorial policy.
  1. Special Projects Office Bureau of Naval Weapons Department of the Navy. "PERT Summary Report Phase 1."

  2. Malcolm, D.G., et al. "Application of a Technique for Research and Development Program Evaluation," Operations Research, vol. 7, no. 5, 1959, pp. 646-669.

  3. Smart Sheet. "PERT Chart vs. Gantt Chart."

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