Schedule Development

PERT Analysis

Program Evaluation and Review Technique (PERT) is a method used to examine the tasks in a schedule and determine a Critical Path Method variation (CPM). It analyzes the time required to complete each task and its associated dependencies to determine the minimum time to complete a project. It estimates the shortest possible time each activity will take, the most likely length of time, and the longest time that might be taken if the activity takes longer than expected.  The US Navy developed the method in 1957 on the Polaris nuclear submarine project.

Purpose of PERT Analysis

PERT Analysis informs Program Managers and project personnel on the project’s tasks and the estimated amount of time required to complete each task. By utilizing this information a Program Manager will be able to estimate the minimum amount of time required to complete the entire project. This helps in the creation of more realistic schedules and cost estimates.

How to Conduct a PERT Analysis

There are two main steps when determining the PERT Estimate. These two steps are:

  • Step 1: Determine optimistic, pessimistic, and most likely estimates
  • Step 2: Calculate PERT Estimate using the PERT Formula

Step 1: Determine optimistic, pessimistic, and most likely estimates

To conduct PERT Analysis, three-time estimates are obtained (optimistic, pessimistic, and most likely) for every activity along the Critical Path.

  • Optimistic Time (O): the minimum possible time required to accomplish a task, assuming everything proceeds better than is normally expected.
  • Pessimistic Time (P): the maximum possible time required to accomplish a task, assuming everything goes wrong (excluding major catastrophes).
  • Most likely Time (M): the best estimate of the time required to accomplish a task, assuming everything proceeds as normal.
Example of the three-time estimates

Step 2: Calculate PERT Estimate

After completing Step 1, use the (optimistic, pessimistic, and most likely) estimates in the formula below to calculate the PERT estimate for the project.

Formula: (P+4M+O)/6

Example: (8 + 4(14)+20)/6 = 14 Weeks

Critical Path

Definition Critical Path: The longest path of scheduled activities that must be met to execute a project.

See: Critical Path Method


Critical Path
Example of a Critical Path Nodal Diagram

Advantages and Disadvantages of PERT Analysis

Understanding the advantages and disadvantages of utilizing PERT analysis will give program managers and project personnel a better understanding of the realities of their schedules. It takes an experienced program manager to truly utilize the benefits a PERT analysis can provide a project team.

  • Advantages: Provides Program Managers information to evaluate time and resources on a project. It helps give them the necessary information to make informed decisions and set a realistic schedule.
  • Disadvantages: The analysis can be highly subjective and be influenced a few outspoken team members. It also required a lot of time to continually update the analysis as a program progresses.

Critical Path Method (CPM) and Program Evaluation and Review Technique (PERT)

Although Critical Path Method (CPM) and PERT are conceptually similar, some significant differences exist mostly due to the type of projects best suited for each technique. PERT is better to use when there is much uncertainty and when control over time outweighs control over costs. PERT handles uncertainty of the time required to complete an activity by developing three estimates and then computing an expected time using the beta distribution. CPM is better suited for well-defined projects and activities with little uncertainty, where accurate time and resource estimates can be made. The percentage of completion of the activity than can be determined. [1]

History of PERT

In 1958, the U.S. Navy introduced network scheduling techniques by developing PERT as a management control system for the development of the Polaris missile program. PERT’s focus was to give managers the means to plan and control processes and activities so the project could be completed within the specified time period. The Polaris program involved 250 prime contractors, more than 9,000 subcontractors, and hundreds of thousands of tasks. [1]

PERT was introduced as an event-oriented, probabilistic technique to increase the Program Manager’s control in projects where time was the critical factor and time estimates were difficult to make with confidence. The events used in this technique represent the start and finish of the activities. PERT uses three-time estimates for each activity: optimistic, pessimistic, and most likely. An expected time is calculated based on a beta probability distribution for each activity from these estimates. [1]

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Updated: 11/14/2021

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