PERT Program Evaluation Review Technique
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 to get a visual representation of a project workschedule 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 will discuss the PERT Method and it’s benefits and limitations.
Differences Between CPM and PERT
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 project involves risks, difficulties and uncertainties. It is a good idea to use the PERT Method in such sophisticated projects.
Below are some 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
• Most Likely Estimate
• Optimistic Estimate
• Pessimistic Estimate
Most Likely Estimate (Tm)
Optimistic Estimate (To)
Pessimistic Estimate (Tp)
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 workschedule.
- 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 workschedule will be effected and incorrect results will occur.
- It is not easy to use the Program Evaluation Review Technique without a help of a software.
PERT Program Evaluation Review Technique Example
Assume that you are managing a software development project. During the meeting, one of your member 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 can not 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 are uncertain. The PERT Method allows to identify the critical path of the project. However, it is not very easy to use. Most of the schedulers find it difficult to implement. Employing softwares like Primavera P6 or Microsoft Project allows to make PERT calculations easier and faster. In this article we discussed the 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.
PERT Method -Program Evaluation and Review Technique Example