What Is A PERT Chart & How To Use Them!

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There are lots of different techniques for managing complex projects. One of those techniques is the PERT charts. If you’re interested in knowing more about these charts, keep reading to find out what they are, how they are used, and their pros and cons.

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Standing for Project Evaluation Review Techniques, the PERT chart was created by the US Navy in the 1950s to manage the Polaris Submarine Missile project.

At the same time, the private sector developed a similar technique called the critical path, which is also well-known in project management.

Both methods are very similar and focus on the essential items that will delay the project from reaching completion.

Illustration Of PERT Chart

A PERT chart illustrates a project as a network of numbered boxes and arrows.

Numbered boxes or nodes represent events in a project.

Lines represent tasks and are labelled with the amount of time allocated to that task.

Arrows that converge on a specific node show the dependent tasks that must be completed before subsequent tasks can be completed.

Similarly, arrows that run in parallel show tasks that can be worked on and completed concurrently, independently of prior tasks.

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How Managers Use PERT Chart

Project managers use PERT charts to get a bird’s eye view of an entire project or subsection of a project.

They make it very easy to identify potential bottlenecks, interdependencies between tasks, and which parts of a project are lagging relative to other parts.

They are similar to Gantt charts but more complex to interpret because they visually represent task dependencies as well.

For this reason, many project managers often like to use both representations in parallel when working on large projects.

Gantt charts are most helpful for project managers who want to visualise how much of each specific task has been completed, something that a standard PERT chart doesn’t include.

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How To Create A PERT Chart

There are five steps to creating a PERT chart.

1. Identify all discrete activities required to complete a project.
As with all project management, the first step is identifying exactly what you would like to achieve.
In this case, you break it down into the largest piece of an area of a project that can be completed start to finish without requiring any input from another part of the project.

2. Identify all dependencies between the tasks in the list above.
This is where things get complicated.
You need to identify for each task what the precedent tasks are that need to be completed before this task can be started.

3. Identify how long each task will take to complete.
Experienced project managers will have multiple ways of looking at this.
A simple method to estimate this is the PERT formula.
This is represented at ( O + (4M) +P ) / 6 = Expected duration.
O is the optimistic timeline, M the most likely timeline, and P is the pessimistic timeline.
For example, if O is 2 days, M is 3 days and P is 6 days. This would give a project length of 3 ⅓ days.

4. All tasks are represented by arrows on the chart.

5. All milestones and dates are represented as nodes on the chart.

Let’s look at the simplest possible case, a single activity.

To represent it, we would have a single arrow running from node 1 to node 2.

On the arrow, we would put the activity and how long it would take. For example, ‘Collect project data: 3 days”. On the nodes we would put the date, so for example on node 1 we could put 5 January and on node 2 we would put 8 January.

Let’s expand this by looking at an example with multiple activities.

Example of a PERT chart with eight activities in total.

In the diagram, the circles are milestone dates.

This makes it very easy to see that the activities will only reach milestones 3 and 6 once milestone 2 is completed.

You can also see that milestone 6 requires input that initiates when milestones 4 and 5 are completed, and that milestone 5 is dependent on milestone 4.

This would be very complex to represent in a table or on most other project management charts.

When To Use A PERT Chart?

The PERT charts are helpful throughout the lifecycle of a project.

Their ability to identify critical paths for projects means that they are very useful in the following circumstances:

1. When planning a project, to determine how long it will take.
2. When trying to understand which tasks can be completed in series and which in parallel.
3. When a project is facing delays, they allow you to quickly identify where delays will cause issues later in the project.

When Not To Use A PERT Chart

PERT charts suffer from a number of weaknesses:

1. They are time-consuming to create and keep updated.
2. Given how time-consuming they are to create, they may not be kept updated with small changes to projects, making them increasingly inaccurate over time.
3. They do not incorporate a financial element in their standard representation. As such, they do not incorporate a key project variable, namely, its financial budget.

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Final Thoughts

PERT charts are part of a project manager’s toolkit.

They can’t be used to solve every problem on a project, but if you are looking at issues relating to a project’s critical path and key dependencies, they are very useful.

About Ben Richardson

Ben is a director of Acuity Training which he has been running for over 10 years.

He is a Natural Sciences graduate from the University of Cambridge and a qualified accountant with the ICAEW.

He previously worked as a venture capitalist and banker and so had extensive experience with Excel from building financial models before moving to learn SQL, Microsoft Power BI and other technologies more recently.