Program Evaluation and Review Technique

PERT Technique: A Comprehensive Guide

Why PERT is Important

Program Evaluation and Review Technique (PERT) is crucial in project management for several reasons:

1. Handling Uncertainty: PERT allows project managers to account for variability in task durations, making it especially valuable for projects with high levels of uncertainty.

2. Risk Management: By using three time estimates, PERT helps identify potential schedule risks early in the planning process.

3. Realistic Scheduling: The weighted average approach provides more realistic time estimates than single-point estimates.

4. Critical Path Identification: PERT helps identify the critical path, enabling better resource allocation and schedule management.

What is PERT?

PERT is a statistical tool used in project management for analyzing and representing the tasks in a project. Developed in the 1950s by the U.S. Navy for the Polaris submarine missile program, PERT is particularly useful for projects where activity durations are uncertain.

Unlike deterministic methods like Critical Path Method (CPM) that use fixed time estimates, PERT uses three time estimates to calculate expected duration:

• Optimistic time (O): The minimum possible time required if everything goes perfectly
• Most likely time (M): The most probable duration under normal conditions
• Pessimistic time (P): The maximum possible time if significant problems occur

How PERT Works

1. Expected Time Calculation
The core of PERT is calculating the expected time (TE) using the weighted average formula:

TE = (O + 4M + P) ÷ 6

This formula weights the most likely time four times as heavily as the optimistic and pessimistic times, creating a beta distribution.

2. Variance Calculation
PERT also calculates variance for each activity:

Variance = [(P - O) ÷ 6]²

This helps quantify the uncertainty in the duration estimates.

3. Network Diagram Creation
PERT uses network diagrams to visualize task dependencies, with nodes representing milestones and arrows representing activities.

4. Critical Path Determination
Like CPM, PERT identifies the critical path—the sequence of tasks that determines the project's minimum duration.

5. Probability Calculations
PERT can calculate the probability of completing the project by a specific date using statistical methods and the normal distribution.

Example Application

Consider a software development task with these estimates:
• Optimistic (O): 4 days
• Most likely (M): 6 days
• Pessimistic (P): 10 days

Expected time (TE) = (4 + 4×6 + 10) ÷ 6 = 38 ÷ 6 = 6.33 days
Variance = [(10 - 4) ÷ 6]² = [6 ÷ 6]² = 1² = 1

Exam Tips: Answering Questions on Program Evaluation and Review Technique

1. Know the Formula: Memorize the PERT formula TE = (O + 4M + P) ÷ 6 and the variance formula [(P - O) ÷ 6]².

2. Practice Calculations: Be comfortable calculating expected times and variances quickly.

3. Understand the Distribution: Remember that PERT assumes a beta distribution for activity durations.

4. Recognize Application Scenarios: Identify when PERT is appropriate (projects with uncertainty) versus when CPM might be better (well-defined projects).

5. Probability Questions: For questions about completion probability, recall that PERT uses the normal distribution and standard deviation (square root of the sum of variances on the critical path).

6. Critical Path Focus: Remember that PERT, like CPM, focuses on the critical path for determining project duration.

7. Know the Differences: Be clear on how PERT differs from CPM (three time estimates vs. single estimate).

8. Conceptual Understanding: Beyond calculations, understand the purpose of PERT in managing uncertainty.

9. Float/Slack: Understand how to calculate float or slack time in a PERT network.

10. Standard Deviation: Know that standard deviation for an activity is (P - O) ÷ 6 and for the project is the square root of the sum of variances on the critical path.

By mastering these concepts and practicing calculations, you'll be well-prepared to answer PERT-related questions on the PMI-SP exam.