Everything you need to know about PERT charts

Everything you need to know about PERT charts

The Program, Evaluation, and Review Technique (PERT for short) is a popular project management tool that dates all the way back to the ‘50s. It breaks down the separate tasks that make up a project, then helps you analyze the time it will take to complete each task. It also helps you identify the minimum amount of time you’ll need to complete the entire project as a whole.

It’s represented as a diagram made up of arrows, rectangles, circles or tables, which map a project’s tasks and the overall schedule. They can be very top-level and show only the most crucial stages or a project, or be more detailed and granular. But at their simplest, they will help you work out the following:

  • Anticipate the amount of time it’ll take to complete a project.
  • Make informed decisions about complex projects.
  • Spot new opportunities for project optimization.
  • Determine a critical path to project completion, including task order.
  • It incorporates uncertainty, so you can schedule a project without knowing precise details and timeframes.

What’s the difference between a PERT and a Gantt chart?

If you’re familiar with Gantt charts, then you’ll probably have spotted a similarity: both diagrams break projects down into smaller tasks. And both deal with the sequence of tasks needed to complete the project.

Gantt charts provide a timeline of smaller tasks, with dates to show task durations and completion times. They’re start- and finish-oriented, and look like a series of horizontal blocks that stretch across a calendar. Gantt charts essentially bar charts and are 100% focused on time.

gantt chart diagram

PERT charts are often used before the project begins to help you estimate more accurately. They’re flow plans that focus on the relationship between different tasks.

pert chart diagram

What do those arrows, circles, and boxes mean?

The rectangles/circles (known as ‘nodes’) indicate each completed phase of the total project. And arrows represent the activities or work necessary to reach each phase (or node). The direction of the arrows shows the sequence of tasks, including whether certain tasks are dependent on others or whether they can run simultaneously.

PERT chart terms you need to know

Critical Path: The longest path from the beginning to the end of the project.
Float or Slack: This refers to the amount of time a task can be delayed before it causes an overall delay or impacts the other tasks.
Critical Path Activity: This is an activity with no slack.
Lead Time: How much time you have to complete a task or activity without impacting the following activities.
Lag Time: The earliest time in which one task can follow another.
Fast Tracking: Performing tasks or activities simultaneously.
Crashing Critical Path: Shortening the duration of critical tasks.

How to create your PERT chart

There’s more than one way to skin a cat, as the (slightly gross!) saying goes. Some people like to do broad top-level charts that map only the most crucial stages. Others like to go deep and stuff in as much info as possible, including dates. Different projects require different techniques.

  1. Choose which kind of network diagram you want to use. Arrows represent activities, but do you want circles or more detailed tables as your node milestones?
  2. Break your project down into tasks. Work out which tasks are required to complete the project, including their individual duration, and the order in which they’ll need to be completed. You don’t want to create an overly beastly chart, so start with the top-level activities. You might want to break these down into more granular tasks later on, but for now: keep it simple.
  3. Build links between tasks. Does the completion of one task depend on the completion of another? Can some tasks be run simultaneously? Note these down.
  4. Designate timeframes. Work out how long it’ll take to complete each task. PERT diagrams commonly utilize three types of time estimates: optimistic (o), pessimistic (p) and most likely (m). These are then used to calculate the expected time (te), which accounts for delays and deviations. There’s even a formula you can use to work this out: te = (o + 4m + p) ÷ 6. Clever, right?

Top tip: The lack of a timeframe makes it harder to show task status, so color-code your notes to show progress and completion.

How to use a PERT chart

So you’ve got your diagram all mapped out — what now? You’ll probably want to work out how long your project will take from start to finish.

PERT diagrams are commonly used in conjunction with the Critical Path Method (CPM), which is essentially a technical-sounding way of saying ‘how long will it take to complete each task before you can finish the project.’You work out your critical path by identifying the longest stretch containing the activities or tasks that will take the longest. Measure the length of time required to complete them from start to finish, and you’ll have your critical path.

Once you have this, you’ll have a pretty good idea how long you’ll need to complete your project. You can use this figure to then choose your start date and end date. You can refer back to your diagram as you go to make sure you’re on track.

Final thoughts

Like network diagrams, PERT charts can be unwieldy, so use a specially-designed diagramming tool, like Cacoo. After all, you don’t want to spend half the time you’ve set aside grappling with formatting. Yes, we’re looking at you, MS Word!

You’ll also want to create something you can share and edit easily, especially if your colleagues and clients are in different offices or countries. Something cloud-based is ideal because it means everything safely stored in one easy-to-access place, which everyone can access wherever they are. Most importantly: pick software that works for you. After all, network diagrams should solve problems, not create them!

Georgina Guthrie Georgina is a displaced Brit currently working in France as a freelance copywriter. Before moving to sunnier climates, she worked as a B2B agency writer in Bristol, England, which is also where she was born. In her spare time, she enjoys old films and cooking (badly).