The Program Evaluation Review Technique, commonly known as PERT, is a visual tool in project planning that helps organizations analyze and represent the activity, and evaluate and estimate the time required to complete the project within deadlines. PERT allows planners to identify start and end dates, and ultimately reduce costs and time needed to complete the project.
PERT was developed in 1958 by the US Navy as part of the Polaris project. Their aim was to manage the Polaris submarine missile program. At around the same time, an American chemical company El DuPont de Nemours devised a very similar approach called Critical Path Method (CPM).
Unlike CPM, which determines the longest path needed to complete a project, PERT gives you three different time estimates. While CPM focuses on time, PERT focuses on time-cost trade-off.
PERT in 5 steps
Step 1. Identifying specific activities and milestones
By listing all your tasks in the table, you get the clear overview of all the steps which you can subsequently expand by adding information on sequence and the time necessary to complete each activity.
Step 2. Determining the sequence of activities
While it easy to predict the order of some activities, other tasks may require more in-depth analysis which will help you determine their order more easily
Step 3. Constructing a network diagram
Once you established the sequence of activities, you can represent both serial and parallel activities in the diagram. Each activity should represent a node in the network, and you can use arrows to show relationships between activities.
Step 4. Estimating the time necessary for each activity
What distinguishes PERT from other techniques is its ability to deal with uncertainty in activity completion time. There are three-time estimates this model typically uses for each activity:
- Optimistic time - the shortest time in which the activity can be completed
- The most likely time - the completion time that has the highest probability
- Pessimistic time - the longest time in which the activity can be completed
After you identified the time estimates, you can calculate the expected time for each activity by using the following weighted average:
Expected time = ( Optimistic + 4 x Most likely + Pessimistic) / 6
For example, imagine you are building a cottage. Drilling and planting the posts has an optimistic duration of 7 hours, an expected duration of 10 hours, and a pessimistic duration of 12 hours. The optimistic duration is counted once, the most likely time is counted four times and the pessimistic time is counted once.
The entire sum is then divided by 6 and the weighted average is 9,83.
You perform this calculation for each task and then insert it into duration column.
Step 5. Identifying the critical path
By adding the times for the activities and determining the longest path, you create a critical path. The critical path involves the total amount of time necessary to complete the project. The total project time doesn’t change if activities outside the critical path speed up or slow down.
At its core, PERT gives you the ability to control complex and ambitious projects whose objectives can be highly critical in nature. It helps you determine the fastest possible route to complete the projects.
PERT gives you a number of benefits:
- In-depth analysis of project activities by viewing the activities both independently and in connection with each other, you get a clear view of the time and the budget required to finish the entire projects.
- What-if analysis helps you identify all the possibilities and uncertainties related to the project. By trying different combinations and choosing the most useful possibility, you eliminate the risk of having project surprises. Also, it helps you highlight the activities that require careful monitoring.
PERT helps project managers identify responsible departments and delegate roles to their team workers. By gathering information from multiple sources, you can easily coordinate project activities and leverage communication between departments. Through efficient planning and decision-making, you encourage your team to truly invest their time and energy into delivering the best results.
Even though PERT has proven to be effective in terms of reducing the expected project completion time, there are still some limitations we need to be aware of:
- Although PERT makes you clearly define all the activities on a project, it’s sometimes impossible to predict every step. Changes happen during the project and they can seriously affect the initial PERT. While it’s possible to make modifications, it takes a lot of time and energy without contributing much to the project.
- Project managers make time estimates and since they heavily depend on judgement, the numbers are only guesses, especially if the project manager has little experience with the activities at hand.
Overall, PERT allows you to have an idea of possible time variation and helps you assess the importance of problems you have to face along the way. Unlike most methods, PERT gives you the flexibility to identify the best-case and the worst-case scenarios and develop a strategy on how to best coordinate large-scale projects.