The PERT Method -Program Evaluation and Review Technique Example
The PERT Method (or Program Evaluation and Review Technique) is one of the most common scheduling methods for network analysis which is used to coordinate complex tasks. PERT Method helps to analyze the work schedule by focusing on each task and calculate the minimum time required to complete the project.
In this respect, it is similar to the Critical Path Method(CPM). Both of them uses common approach for preparing the network and determining its critical path. But there are some differences between them.
What is PERT ?
PERT Method was first developed by the US Navy SPO (Special Projects Office) in 1967 during the Polaris missile development program then it was applied to the other industries. Both CPM and PERT are complementary tools and they are developed at roughly the same time. Unlike other methods, PERT is an event oriented technique which uses three time estimates (optimistic, expected, and pessimistic) for each task. With this aspect it employs uncertainty while calculating the task durations. These characteristics makes the PERT Method useful for large-scaled, complex and non-routine projects.
The WBS (Work Breakdown Structure) concept which divides the scope into manageable sections was developed in conjunction with the PERT Method.
Key Differences Between the PERT Method and the CPM (Critical Path Method)
Below are some key differences between the PERT Method and the Critical Path Method
– In CPM Method activity durations are well defined but in PERT Method , time estimations are uncertain.
– CPM is a method of control of time and costs, however PERT is a technique of planning and control of time.
– There are three estimates in PERT such as optimistic, pessimistic and most likely. In CPM Method, there is only one estimate.
– In PERT diagram, only FS relationship type is applicable. In CPM diagram 4 relationship types (FS,SS,SF,FF) can be applicable.
– PERT is event-oriented however, CPM is start and completion oriented.
– PERT is more applicable to research and development projects rather than CPM.
Basics of the PERT Method
The PERT method employs simple statistic calculations. It uses three time estimations.
- Optimistic Estimate : The shortest time required to complete the task.
- Pessimistic Estimate: The longest time required to complete the task.
- Most Likely Estimate: The most possible time (probable duration) required to complete the task.
Expected time is calculated by the help of the formula below
Expected time = ( Optimistic + 4 x Most likely + Pessimistic) / 6
The PERT Method Implementation Steps
1. List the activities and milestones
First step is to determine the tasks required to complete the project.
2. Determine the sequence of activities
Second step is to determine the order of the activities. Which activity is predecessor which one is successor. It is easy to determine the sequence of some activities however the sequence of some tasks may require deep analysis.
3. Build a network diagram
Third step is to create the network diagram by the help of a software or by hand and place the activities on the diagram.
The PERT Method uses three duration estimates for activities which are;
- Optimistic Estimate
- Pessimistic Estimate
- Most Likely Estimate
By the help of three estimates, expected duration is calculated.
5. Determine the critical path
The PERT Method Example
Let’ s assume that you are a project manager of a power plant project and you listed;
– All the Activities
– Optimistic, Pessimistic and Most Likely Activity Durations
By using “The Pert Formula = (To + 4Tm + Tp)/6”, you calculated the expected duration for each activity.
All the inputs are listed in the table below.
The associated network is below.
After building a network diagram and estimating the activity durations, you will determine the critical path by making forward and backward pass calculations.
Forward Pass Calculations specify the minimum dates at which each activity can be performed and, ultimately, the minimum duration of a project.
Backward Pass Calculations determine the latest dates by which each activity can be performed without increasing the projects minimum duration.
After completing the backward pass calculation, you can easily determine the critical path. In project management, “float” or “slack” is the amount of time that a task can be delayed without affecting the deadlines of other subsequent tasks, or the project’s final delivery date. Total float/slack is 0 on the critical path.
Total Float: LS – ES = 18-15 = 3
Total Float: LF – EF = 30-27 = 3
Total float can be calculated by subtracting the Early Start date of an activity from its Late Start date or Early Finish date from its Late Finish date.
When we analyze the network diagram we will see that there are some paths and every path have duration.
The critical path is the longest path in the network diagram and total float of critical path is zero.
In the real life, projects are not executed as it was planned all the time. Risks, unexpected events, uncertainty and subjective estimates are the significiant reasons of deviations. PERT is helpful for estimating the project completion time in case of uncertainty. However it may be difficult to use without a software.
In this example we calculated the expected duration for each activity. By making forward, backward calculations, we determined the critical path. Accuracy of expected durations which effect the critical path, depend on the optimistic, pessimistic and most likely duration estimates. By the help of PERT softwares it is easy to make these calculations efficiently for many combinations.