What is a PERT Chart?
PERT is an acronym that stands for Program Evaluation and Review Technique. It’s a statistical method that can be very useful when working on a project, as it analyzes and represents the project’s tasks.
A PERT chart is a graphic representation of the data you generate from that method, laid out as a timeline. It’s a critical tool project managers can use when putting together a project schedule, as it allows them to break down each of the project’s tasks for analysis.
The purpose of the PERT chart is to find out how much time will be required to finish each task in the project. This leads to figuring out the minimum time needed to complete the whole project. What’s unique about a PERT chart is that you don’t have to start your plan with a beginning and an end. Since it’s focused on events, you can create one without knowing every detail.
A PERT chart also gives you the raw data that can be imported into project scheduling software, which helps you manage the whole project, link your schedule to your plan, monitor progress and keep stakeholders in the loop with reports.
ProjectManager.com makes harnessing your PERT chart and tracking your critical path simple. Sign up for a free trial today and make dynamic project schedules that are easy to edit and update in real time.
Why Use a PERT Chart?
The PERT chart is used to estimate the minimum amount of time that will be needed to close a project. This is done by examining the breakdown of the project and the connections between tasks, which helps determine the amount of risk inherent in the project.
Simplify Complex Projects
PERT charts help project managers get a handle on complex projects. They’re useful because they provide an estimation of how much time you’ll need to schedule the project and what resources will be required. The nature of the PERT chart and its breakdown structure helps to visualize the complexity of a project and the dependencies between each step in the process.
Involve Multiple Departments & Subject Matter Experts
A PERT chart also serves as an organizational tool that pulls in data from every department working on the project. This helps identify who is doing what, and provides clear responsibilities for those tasks. Strong communication is facilitated between departments, and the overall strategic goals of the organization are better aligned.
Explore the Hypothetical
Another benefit of using a PERT is it allows you to create what-if scenarios. By mapping out the time and resources for a project, you can quickly see what is working and what is not. This saves a lot of time that would otherwise be wasted in revisions.
Pitfalls of PERT
PERT charts are not a panacea to get your projects magically on track, though. Naturally, bad data-in will result in bad data-out. It’s important that the person creating the PERT has experience in the process.
They’re also labor intensive, which some organizations might not have the time or money to invest in. Additionally, you just don’t build a PERT and call it a day. They require continuous reviews and updates to make sure they’re accurate.
PERT Charts in Project Management
As stated, the use of the PERT chart gives project managers a tool to estimate the time and resources needed to complete their project, which is crucial during the initiation and planning phases.
When it comes to starting a project, the early steps involve taking all those tasks you collected that lead to your final deliverable and organizing them in a schedule. Knowing the amount of time it’ll take to complete each task, especially the riskier ones, is fundamental to creating a viable schedule.
PERT Charts vs. Gantt Charts
While both PERT charts and Gantt charts are visual tools used by project managers to control tasks scheduling, they are not exactly the same thing.
PERT charts, as detailed above, were developed to simplify planning and scheduling larger and complex projects. A Gantt chart, on the other hand, is also a graphical depiction for planning and scheduling a project, which breaks down tasks down into tasks that populate a timeline. A Gantt chart can set task dependencies and shows the duration of each task.
There are other key differences, as well. For example, in a Gantt, the timeline is represented by a bar chart. A PERT chart is more of a flowchart or network diagram. Gantts can be used on smaller projects, while PERT charts are for larger and more complex projects. Dependent tasks are linked on a Gantt, while a PERT has many interconnecting networks of independent tasks.
The most salient difference is that a PERT chart is usually used before starting a project to figure out scheduling, while Gantt charts tend to follow into the project, highlighting scheduling constraints. Gantt chart software can be used when executing the project as well as when planning because a task’s start and end dates can be edited. Gantt charts reveal how long each task will take, show who on the team is responsible for those tasks and generally are a more transparent tool to track progress.
Pro Tip: A PERT chart is a great way to help you estimate simply and more accurately. Estimates can be overly optimistic or pessimistic, but using a PERT chart will find the most realistic estimate.
Steps to Implementing a PERT Chart
Use a PERT in the planning phase of your project. Here are the steps in broad strokes:
- Begin by identifying the project milestones and then break those down into individual tasks.
- Figure out the sequence of the tasks.
- Make the PERT diagram — we’ll show you how in the section below!
- Do an estimate for each task and the time it will take to complete it.
- Calculate the critical path and identify any possible slack.
- You have your PERT chart! Remember, the PERT chart is a living document that must be returned to and revised as needed as the project progresses.
How to Calculate PERT
Before we create a PERT chart, it’s helpful to know how to calculate PERT itself.
PERT relies on the weighted average of three numbers that are based on the most pessimistic (P), the most optimistic (O) and the most likey (M) estimates for the project’s length.
- Optimistic Time: The least amount of time to accomplish a task or activity. This is a scenario when everything is working and you beat the estimated schedule.
- Pessimistic Time: The maximum amount of time to accomplish a task or activity. This is the worst-case scenario, anything that can go wrong does.
- Most Likely Time: The best estimate of how long it will take to accomplish the task or activity, assuming there are no problems. This would be your estimation working out perfectly, as rare as that might occur.
- Expected Time: The best estimate of how long it will take to accomplish the task or activity, assuming there will be problems. This would be the more realistic duration.
Using the figures you come up with for P, O and M, calculate the equation:
(O + 4M + P)/6
To determine the volatility of your estimate, subtract the pessimistic number from the optimistic one and divide the results by six. The larger your results, the less confidence you have in your estimate, and vice versa; the smaller the figure, the greater your sense of confidence in the estimation.
The result is a weighted average, which is an average from multiplying each element by a factor that reflects its importance. You can think of this as the expected time, though often the calculation will bend towards the pessimistic. That’s because usually the pessimistic figure is larger than that of the optimistic one.
This calculation helps you estimate all the tasks in your schedule, but it can also focus solely on activities that are high risk. The latter is helpful as the more risky the task, the wider the margin of error is when making an estimation of how much time it’ll take.
Creating a PERT Chart
Let’s move these concepts from abstraction to reality. To better understand the power of a PERT chart, let’s make one together. For our PERT chart example, we’ll create a project around building a website. The PERT chart will help us organize our project with milestones, allow us to estimate our tasks and quickly uncover the critical path.
Observe the PERT Chart below, and we’ll walk through how we created it.
PERT Chart Example
To begin, we’re going to list the milestones for our website project. Keeping this process simple for the sake of clarity, here are eight project phases.
- Create Copy & Art
- Build Site
- Develop Marketing
- Test Site
- Edit Copy & Art
- Marketing Push
First, we’ll transfer these to the PERT chart. Each phase is turned into a node, which is displayed as a circle with numbers in them; these will be our project milestones.
Next, we figure out how long all the tasks to get to our deliverables will take. The amount of time you estimate should be added to the chart next to each node. In our example chart, the time estimate is depicted by small green circles in the corner of each node. We used weeks as our time unit, but it could be days or months, depending on your project.
Once this is done, we link up the milestones using vectors. Vectors are the lines that have arrows—these arrows represent the flow and sequences of events that will take the project from start to finish. More than one arrow can go to more than one node. As in our example, the creation of copy and art will go towards the software team building the site, but also goes to the content team who will be editing that work.
NOTE: A vector that is solid needs to take place concurrently. If the vector is dotted, however, it means that the work must be done in sequence, but doesn’t require resources.
With the completed chart, it becomes clear which activities are critical to delivering the project on time. These are represented by the red numbers in the nodes. Once the critical path is determined, you can easily refer to it to keep your project on track!
Glossary of Terms Related to PERT Charts:
- Nodes: These are the symbols used to visualize milestones and project tasks. They can be represented by circles or triangles.
- Vectors: Visual representation of the sequence of a task, diverging arrows indicate tasks that can be completed at the same time. They can be solid or dotted, depending on the nature of the sequence.
- PERT Event: The start or end of a task.
- Slack: The amount of time a task can be delayed without causing an overall delay to the project or other tasks, also known as float.
- Critical Path: Charts the longest path from beginning to the end of a task or event.
- Critical Path Activity: An activity with no slack.
- Lead Time: How much time you should complete a task or activity without impacting the following ones.
- Lag Time: The earliest time in which a task can follow another.
- Fast Tracking: Working tasks or activities at the same time.
- Crashing Critical Path: Shortening the time of a task. This is the longest path from the starting milestone to the finish. If you experience a delay on the critical path, it will impact the entire project.
PERT Chart Video
All of these explanations can be a bit hard to understand by text. If you’re more of a visual learner, watch project management expert Jennifer Bridges, PMP, as she walks you through the steps of creating a PERT chart in this short tutorial video.
History of PERT Charts
PERT in project management has been around for a while, but it was, in fact, developed by the U.S. Navy. In 1957, the Special Projects Office created the PERT chart to assist in its Polaris nuclear submarine project.
Since then, it’s found a home in all manner of industries, even the 1968 Winter Olympics in Grenoble.
PERT diagrams and the critical path method came about at roughly the same time, growing from the scientific management founded by Frederick Taylor, also called Taylorism, which was later refined by Henry Ford. But the use of the term critical path comes from DuPont, which developed the method also in the late 1950s.
Moving Beyond PERT with ProjectManager.com
The PERT chart is a tool that helps plan and schedule. ProjectManager.com helps you take that PERT chart and apply it to managing your whole project with our online Gantt chart. It plans, schedules and works through the execution of the project. Our Gantt gives you control over every aspect of your schedule, from placing milestones, assigning team members and linking dependencies.
We easily link all four types of task dependencies — finish-to-start, finish-to-finish, start-to-finish and start-to-finish — so you can avoid bottlenecks later in the project.
Unlike a PERT chart, our Gantt isn’t static. No matter how good your estimations, there will always be issues that pop up in a project. You can prepare for risk and mitigate it, but often the schedule will have to adapt to keep you on time. We have drag-and-drop editing to quickly and easily change start and due dates, which is then reflected throughout the software.
That’s just the tip of the iceberg for our award-winning project management software. We can help you keep your projects and teams organized. See for yourself why tens of thousands of teams use our software to work more productively by starting your free 30-day trial.
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