PERT Diagrams: Definition and Use for Project Planning

By Indeed Editorial Team

Updated March 30, 2022 | Published December 12, 2019

Updated March 30, 2022

Published December 12, 2019

Program Evaluation Review Technique (PERT) diagrams help provide structure, time estimates and project resources required for large projects. PERT diagrams are an essential tool for project managers who want to utilize methods of staying on time and on task toward meeting their objectives. They provide insights on important metrics like the estimated time it takes to complete an activity, the time it takes to complete a project and how much room there is for setbacks.

This article will detail what a PERT diagram is and how to create one.

What is a PERT diagram?

A PERT diagram, also known as a PERT chart, is a tool used to help project planners track project timelines and budgets, analyze project activity and monitor the progress of projects against their deadlines. It is highly visual and uses objects connected by simple lines to assist with project management workflow.

A PERT diagram can provide you with an idea of potential timeline fluctuations as well as possible roadblocks and a plan to overcome them. It is designed to give a certain amount of flexibility to a project timeline by considering the best- and worst-case scenarios of each step. This helps project planners to strategize and coordinate large-scale projects.

How to create a PERT diagram

Creating a PERT diagram can be done by following these steps:

1. Identify overall project objectives, short- and long-term goals

Start by defining what you want the project to accomplish, differentiating between the short- and long-term goals if necessary. Then create a list of all the tasks necessary to complete the project. This will give you a clear outline of the project as a whole and a breakdown of all the steps you will take to reach the end goal. You can expand each step with relevant details such as the projected sequence and timeline of each task.

Related: SMART Goals: Definition and Examples

2. Determine the series of steps needed to reach project goals

Taking the time to determine the order in which the project tasks should be completed may require a closer look at the details of each task. Some tasks will be easier than others to place in the overall sequence, but keep in mind that the plan may shift along the way as the project evolves.

3. Create a network diagram connecting each step toward project completion

The network diagram is a chart of flows and hierarchies. It contains labeled objects, like boxes, circles or ovals, connected by thin lines. The lines demonstrate the relationships between two objects, the order they should be completed and whether or not two things can be worked on at the same time. Like many flow charts, the lines denote the relationships between two or more activities. In this case, all activities serve to complete the goal of project management.

A practical way to create this body of the diagram is to start by writing the project objective at the top of the page and listing all of the steps that need to be completed in order to reach the goal. Then attach who should complete them and reorganize them into groups that determine which tasks need to be completed first, second, third and so on. There are a number of templates and examples that can be used to help you organize your PERT chart by searching online and filling in your list items.

Related: Workplace Continuous Improvement Plan: Definition, Techniques and Examples

4. Estimate the timeline of each step and complete the diagram

PERT makes allowances for a certain amount of guesswork in activity completion time. The time estimates this model typically uses are:

  • Optimistic time: the shortest amount of time in which the step can be completed

  • Most likely time: this completion time is probably the most accurate

  • Pessimistic time: the longest amount of time that can be allowed for each step

Once you’ve determined the time estimates, use the following weighted average to calculate the expected time for each step:

Expected time = (optimistic + 4 x most likely + pessimistic) / 6

This formula gives more weight to the most likely amount of time it will take to complete the project. Perform this calculation for each task, and insert it into the corresponding time duration part of your network diagram for each step in the process.

For example, consider a project in which your team is developing a new inventory tracking system for your company. If everything goes as it should, the project will take 60 hours to complete. In the most pessimistic scenario, the project will take 120 hours. Considering that both challenges and favorable opportunities will arise, you could consider your most likely completion time to be 90 hours. Using these figures, your formula would look like this:

Expected time = (60 + (4×90) + 120) / 6

After completing this formula, the expected project completion time would be 90 hours.

How to use a PERT diagram

PERT diagrams provide useful insights that help project managers organize and delegate work, create deadlines and milestones and ensure the project completion is on track with a quick, easy visual and a couple of important calculations. Here are some ways to use a PERT diagram:

Identify critical path and project milestones

A completed PERT diagram helps you identify the critical path—an important metric for project managers to use for the creation of project deadlines. This is one essential function of a PERT chart. It makes up the last step, which requires you to gain insight from your newly developed PERT model.

The critical path is a measure of time that estimates the “time to completion” target number of your entire project as it is written in your PERT chart. With the information you’ve already gathered on your completed chart—steps that need to be taken to complete the project and expected time calculation—you should be able to determine the critical path by adding the expected time together. The sum of the expected time calculations at each step is the critical path for the entire project.

Using the critical path method, or CPM, you can further analyze your chart, developing project milestones from the insights gathered.

For example, say your critical path takes one year from project start to finish. You may want to establish milestones every three months to deliver results and report on progress to stakeholders. By grouping tasks into expected delivery based on the expected time values listed, you can set milestones that can be completed at three months, six months, nine months and project completion.

Related: Decision-Making Methods for the Workplace

Extrapolate slack time requirements that keep you on track

Slack time is a term that defines the amount of time an activity can start beyond the predicted start date to ensure the project is delivered on schedule. Basically, if a project is running behind schedule, your slack time gives you a little leeway to start essential activities in the project sequence later than expected while still meeting the overall project delivery deadline.

Slack time can be calculated as follows:

  • Pessimistic time – optimistic time = slack time

For example, imagine one of the essential duties you must complete in a project is to send an email blast. You expect it to take three weeks, optimistically, and six weeks, pessimistically, to design, organize content, complete your email list and push send. You would calculate 6-3=3. That means your slack time is a period of three weeks after you subtract six from three.

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