Program Evaluation Review Technique – PERT

Program Evaluation Review Technique - Pert

Activity on Node and Activity on Arrow Diagrams, Critical Path Analysis, Critical Chain Method are widely used scheduling tools in project time management. A PERT chart is a project management method that allows getting a visual representation of a project work schedule which is used to coordinate and organize the activities of a project.
PERT Program Evaluation Review Technique was developed by the US Navy in the late 1950’ s for Ballistic Missile Program in order to find a simple system to manage and organize complex objectives and thousands of contractors. In this article, we analyze a program evaluation and review technique example for better understanding.

What is PERT and CPM ?

PERT is a statistical method which is used to develop a project schedule when the activity durations are not exactly known. While developing schedules in large and complex projects, schedulers may have difficulty in estimating activity durations because of the lack of historical records. Or the nature of the project involves risks, difficulties, and uncertainties. It is a good idea to use the PERT Method in such sophisticated projects.

In some ways, the PERT Method is similar to the Critical Path Method (CPM). Although both of them are used to create work schedules, they are different.

Differences Between CPM and PERT

  • The Critical Path Method (CPM) focuses on activity however the PERT focuses on event.
  • The Critical Path Method (CPM) is suitable when the activity duration estimates are exact. However PERT is suitable when the activity durations are not exactly known.
  • Activities are represented as nodes in the CPM. On the other hand, activities are represented as arrows in the PERT diagram.
  • There is only one type of relationship which is FS (Finish to Start) used in PERT diagrams. However, in CPM diagrams four activity relationships (FS, SS, FF, SF) can be used to represent the dependencies between the activities.

Experts widely use PERT Method in research, software development, and engineering projects because they can not estimate the duration of each activity easily. On the other hand, the Critical Path Method provides a more deterministic model than PERT.

Estimate Activity Durations in PERT Program Evaluation Review Technique

There are there estimate techniques in PERT Method to determine the duration of activities.

• Most Likely Estimate
• Optimistic Estimate
• Pessimistic Estimate

Most Likely Estimate (Tm)

The most likely estimate is an estimate of the most probable duration of a particular activity.

Optimistic Estimate (To)

The Optimistic estimate is an estimate of the shortest duration of activity that takes into consideration the known parameters that can affect performance.

Pessimistic Estimate (Tp)

The Pessimistic estimate is an estimate of the longest duration of activity that takes into consideration all unfavorable conditions that can affect performance.

The PERT (Program Evaluation Review Technique) Formula

Blow formula is used for PERT Estimate

PERT Estimate = (To + 4 x Tm + Tp)/6
Standard Deviation = (Tp – To)/6

Benefits of PERT Program Evaluation Review Technique

The Program Evaluation Review Technique provides many benefits to the experts. Below are a few of them.

  • Forces project teams to organize the project information with a graphic display.
  • Improves the quality of planning for decision making within the project team.
  • Enables to accelerate projects by the help of efficient planning of activities.
  • Enables to determine the critical path of the work schedule.
  • Enables to define and visualize dependencies between the activities in the network diagram.
  • Enables to calculate slacks which helps project teams to allocate resources effectively.

Limitations of PERT Program Evaluation Review Technique

Following are the few limitations of the Program Evaluation Review Technique:

  • It is less effective for smaller projects
  • It is a complicated method which makes hard to analyze the results.
  • It is a time and cost consuming method because of its complexity
  • Inputs must be unbiased and accurate because this method depends on these estimates and inputs. Otherwise, the work schedule will be affected and incorrect results will occur.
  • It is not easy to use the Program Evaluation Review Technique without the help of a software.

PERT Program Evaluation Review Technique Example (PERT Example)

Assume that you are managing a software development project. During the meeting, one of your members told that creating the project plan will take 15 days to be completed. You analyzed the situation and estimated that depending on the circumstances it may take 25 days in the worst case. However, if all the inputs are available it may take 10 days to be completed.

So you decided to perform the PERT estimate

Most Likely Estimate (Tm)  : 15
Optimistic Estimate (To)     : 10
Pessimistic Estimate (Tp)    : 25

PERT Estimate = (To + 4 x Tm + Tp)/6 ;

PERT Estimate = (10 + 4×15 + 25)/6 = 15.833

Standard Deviation = (Tp – To)/6;

Standard Deviation = (25 – 10)/6 = 2.5


The PERT Method is a powerful tool for implementing “what if” scenarios and is practical in planning for possible obstacles. It creates a roadmap for sophisticated projects in which the duration of activities cannot be clearly estimated. Therefore experts create PERT Charts from back to front because in many projects end date is definite but the duration of major elements is uncertain. The PERT Method allows identifying the critical path of the project. However, it is not very easy to use. Most of the schedulers find it difficult to implement. Employing software like Primavera P6 or Microsoft Project allows making PERT calculations easier and faster. In this article, we discussed the importance of Program Evaluation Review Technique and key aspects of this tool. We hope that it will be useful for project teams and project managers. Note that this is an important concept in project scheduling and planning.

External Reference

Pert Probability

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