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“Predicting Accuracy of Early Cost Estimates Based on Estimate Quality”,

HOW PERT RELATES TO CRITICAL PATH METHOD
In 1957 (around the time the Navy was creating PERT) DuPont developed another project management technique, Critical Path Method (CPM). CPM was designed to address similar challenges it faced in planning how to shut down chemical plants for maintenance and then restart them. For a complete introduction to CPM, read this guide.

The critical path is the sequence of activities in the project with the longest duration. A delay in any of these will push the whole project back. CPM and PERT are often used together, and they both address how long a big project will take. They each have their strengths, and more project managers are using both techniques or merging them.

However, there are some crucial differences between PERT and CPM. The most important is that while PERT focuses on controlling time, CPM focuses on cost optimization. In other words, PERT assumes cost varies directly with time while in CPM cost is not directly proportional to time.

Other distinctions between the two methods are summarized below.

HOW TO CREATE A PERT CHART
How to Create a PERT Chart
Here are the steps to put PERT into practice on your project. There are software programs that can automate the calculations and generate PERT charts.

1- List all the tasks required for your project and the order of completion. Some tasks may be able to happen simultaneously (called non-dependent or concurrent tasks) while others cannot begin until a predecessor task is finished (called dependent or sequential).

2- For each task, give three time estimates in days: the most optimistic completion time (O), the normal/most likely time (M), and the pessimistic time (P).

3- Calculate expected time (TE) using the formula (O + 4*M + P) ÷ 6 = TE. Use our quick calculator

PERT Estimate Calculator
Optimistic Value
Most Likely Value
Pessimistic
See the examples below to understand how to calculate TE.

4- Once you calculate TE, you can create a Gantt chart, a way of visualizing a project schedule using horizontal bars representing time duration across a calendar.

  1. Now you can draw your PERT chart. If you are doing this by hand, you will likely need to work through a few drafts. PERT charts are most commonly network diagrams in which the tasks or major activities are nodes (boxes) with arrows connecting them.
  2. Draft out the tasks. It can be helpful to create a card or box for each task containing the activity name, duration, slack (read on for details on this), start and end dates.