Last Updated on 10 September 2023
PERT stands for Program Evaluation and Review Technique, and a PERT chart (sometimes PERT diagram) is a time planning method for your project. Its main advantage over Gantt charts and CPM in that it allows for the fact that in the real world, project tasks take a variable amount of time, which the two other methods don’t allow for.
In any project, before you perform a stage you don’t know how long a step is going to take, as there are always unknowns (uncertainty) to take into account, including issues that aren’t initially visible.
PERT uses probability to find the ‘best estimate’ to use for each project stage for planning purposes. It then organizes it into a project overview that helps analyze how much spare ‘slack’ there is in the tasks, much like the Critical Path Method.
It was developed by the US Navy in the 1950s to help manage the Polaris submarine missile program.
When do you use them?
PERT charts are a project time planning tool, and are therefore most useful during the planning part of Project management, such as during the Define phase of DMAIC in Six Sigma.
To calculate your best estimate you need to first calculate three figures:
- Most likely: this is the length of time you will use in Gantt or CPM and is your best estimate of the duration of the step
- Optimistic: how quickly the task could be achieved if everything goes well
- Pessimistic: how long the task could take if a lot of things go wrong in the project
The formula used in the PERT method is:
Expected time = (4 x Most likely + Pessimistic + Optimistic) / 6
This is weighted towards the most likely time, but also takes the pessimistic and optimistic estimates into account
Activity Standard deviation
There is also a measure in PERT called ‘Activity Standard Deviation’. It’s important to note that this is different to the Standard Deviation used in statistics and the rest of Six Sigma.
Activity Standard Deviation = (Pessimistic – Optimistic) / 6
This is a measure of how much potential variation there is in the estimate
If I’m doing the ‘Measure Phase’ and I want to calculate the Expected time for how long it’s going to take. For my project I’ve estimated:
- Pessimistic time is 15 days
- Most likely is 9 days
- Optimistic is 6 days
The calculation is then:
Expected time= (4 x Most likely + Pessimistic + Optimistic) / 6 = (4 x 9 + 15 + 6) / 6 = 9.5 days
Standard Deviation (Pessimistic – Optimistic) / 6 = (15 – 6) / 6 = 1.5 days
Issues that affect how long the project steps will take
- Efficiency and experience of the staff carrying out the step
- Illnesses in the team
- Availability of resources
- Political issues
How do you make a PERT Chart?
So far we’ve looked at how to do the PERT calculations, but these are only the first step of creating the PERT diagram.
Step by step guide:
1 – Identify all the steps in your process
There are usually several activities that need to be achieved for the project / stage you are analyzing. Make a list of all the ones that need to be achieved (using brainstorming if necessary) and put them into a data table with a title or concise description. It is usually easier to label them e.g. A, B, C etc for easy reference.
2 – Estimate your Pessimistic, Most likely and Optimistic times
Before you can calculate your expected time, you need to estimate the three times in the calculation: Pessimistic (worst case), Most likely and Optimistic (best case) times for completion of the project step. This will be calculated from your knowledge of the scope of the project, the resources available to you, the complexity and previous experiences of carrying out the steps. It’s a prediction step so you just have to get them as accurate as you can; there is no ‘right answer’.
Put these in your data table next to the project steps.
3 – Calculate your Expected time for each of these steps
This is the calculation that we’ve gong through above. For each of your steps, calculate the Expected time, using the formula
Expected time= (4 x Most likely + Pessimistic + Optimistic) / 6
Add these figures into your data table.
4 – Add the dependencies between the processes
If steps can’t be started until other steps have finished, write the dependencies into the table.
5 – Create your process map using these processes
You’re now ready to create your PERT chart.
6 – Calculate your critical path
As in the critical path method, you can now create your critical path, of those tasks with no slack. The variation in the length of these tasks will affect the time at which the project will be completed.
7 – Keep the chart live
You have now created your PERT chart, but this can be updated as the project develops and you get more information on the project timings. This will keep it up to date and useful for planning the project completion date and giving you early notification of issues.