Using the Critical Path Method (CPM)

A project usually consists of multiple activities, which occur simultaneously and sequentially. To determine the flow of these activities, you’ll need to create a Precedence Diagram. After creating the Precedence Diagram, you can identify the activities, which if delayed, will make your project come in late. The sequence of these activities on the Precedence Diagram is known as the Critical Path. A delay in any of the critical path activities will delay the entire project, regardless of whether the other project activities are completed on or before time.

To determine the Critical Path, you need to:

1. Define the duration of each activity.
2. Identify all the paths.
3. Calculate the duration of each path.
4. Identify the longest path.

To understand this better, let’s consider an example.

You are managing the construction of a shed. You have created a Precedence Diagram and defined the estimated duration of each activity, as shown in the image below. (Click any image in this article for a larger view.)

The Precedence Diagram shown in the image has three paths, as shown:

• Path 1: Start > Purchase Plot > Select Design > Purchase Wood > Assemble Shed > End
• Path 2: Start > Purchase Plot > Select Design > Purchase Paint > Assemble Shed > End
• Path 3: Start > Purchase Plot > Select Design > Hire Workers > Dig Foundation > Lay Foundation > Harden Foundation Current > Assemble Shed > End

To calculate the duration of each path, you can simply add the duration of each activity in each path. For Path 1, you get:

Duration for Path 1 = Duration of Start + Duration of Purchase Plot + Duration of Select Design + Duration of Purchase Wood + Duration of Assemble Shed + Duration of End

Now, plug in the numbers and you get:

Duration of Path 1 = 0 days + 5 days + 3 days + 3 days + 8 days + 0 days = 19 days

Note: The duration of Start and End is zero because there aren’t activities, rather they are events.

Similarly, you can calculate the duration of the other paths:

Duration of Path 2 = 0 days + 5 days + 3 days + 1 days + 8 days + 0 days = 17 days

Duration of Path 3 = 0 days + 5 days + 3 days + 4 days + 4 days + 6 days + 8 days + 8 days + 0 days = 38 days

The longest path is the critical path. From the above calculations, we can see that Path 3 is the longest. Therefore, if activity on this path is delayed, then the project will be delayed. After identifying the critical path, you can deduce the activities that when delayed will not impact the project.

For example, do you think the project will be delayed if the “Purchase Wood” activity is delayed by five days? The answer is no the project will not be delayed. What if the “Purchase Wood” activity came in 8 days late? Read Calculate the Float of Each Activity Using the Critical Path Method (CPM) to determine the amount of time an activity may be late without it delaying the project.