Critical Path Method (CPM)

All tasks are born equal. But some are more important than others.

Critical Path has been beaten to death in many articles on the Internet. But most project managers I interact with still wonder what it really means. This article attempts to bring in some “critical appreciation” on this topic. We’ll understand what it means and why it is important.

What is a critical path?

A critical path is the sequence of tasks that cannot suffer any delay without harming the project delivery date.

In other words, if any critical task is delayed, it will delay the project.

Let’s try to identify critical and non-critical tasks through a simple example

An Example

Let’s say you decide to enclose a part of your garden and build a small room. You hire Bob the builder and the professional that he is, he presents you with the following plan:

For the uninitiated, the above representation is called a Network Diagram. It shows the tasks and their dependencies. A ⟶ B means that B can start only after A is finished.

So let’s understand what the network diagram means:

• There are four tasks – Building, Painting, Flooring, and Decorating.
• Painting and Flooring can only start after Building is complete.
• Painting and Flooring can happen at the same time.
• Decorating can start only after Painting and Flooring are both complete

Bob prepares the following schedule:

Task	Start	Finish
Building	1	5
Painting	6	10
Flooring	6	8
Decorating	11	12

Let’s understand the schedule.

The task “Building” takes 5 days to complete (refer to the network diagram above). So if it starts on day 1, it will finish on day 5.

Since the tasks “Painting” and “Flooring” can start only after the task “Building”, they can start on day 6.

The task “Painting” requires 5 days to complete so it will finish on day 10.

The task “Flooring” requires 3 days to complete so it will finish on day 8.

Since Decorating can only start after both “Painting” and “Flooring” are complete, it can only start on day 11. And since it takes 2 days to complete, it will finish on day 12.

So the project is scheduled to finish on day 12.

Now let’s take one task at a time and see how delaying it affects the end date of the project.

Scenario #1: The task “Building” is delayed by 1 day

Since “Building” will finish a day later, both “Painting” and “Flooring” will start 1 day later so they will finish 1 day late. So “Painting” will finish on day 11 while “Flooring” will finish on day 9.

Consequently, “Decorating” can start only on the day 12 and hence finish on the day 13.

The result: Project is delayed by 1 day.

Scenario #2: The task “Painting” is delayed by 1 day

It will finish 1 day later i.e. on the day 11.

Flooring will continue to finish on time.

Since “Decorating” can only start after both are complete, it will start on day 12 and hence finish on the 13th day.

The result: Project is delayed by 1 day.

Scenario #3: The task “Flooring” is delayed by 1 day

“Flooring” will finish on day 9  instead of day 8.

“Painting” will continue to finish on day 10.

“Decorating” will continue to start on day 11 and finish on day 12.

The result: Project finishes on time!

Scenario #4: The task “Decorating” is delayed by 1 day

The result: Project is delayed by 1 day.

In the above example, you can conclude that other than the task “Flooring”, if any task is delayed, it will result in the project getting delayed. The “Project Manager” way of saying this is that the tasks “Building”, “Painting” and “Decorating” are on the critical path.

The task “Flooring” is not on the critical path In fact, if you look closely, even if it is delayed even by 2 days the project would still finish on time. This “2 days” is called the “free float” or “slack” of the task.

Why is Critical Path useful?

When a project manager manages hundreds or thousands of tasks, it becomes difficult to monitor all tasks on a regular basis. It would be easier to keep a close watch on these critical tasks because remember,  even if one task slips by a day, it would cause the project to be delayed.

If a task that is not on the critical path and delayed, you have to ensure that the slip doesn’t exceed the free float.

How to identify and monitor Critical Tasks?

In the days before computers and project management tools were ubiquitous, all calculations would be performed by hand. At the end of the day, clerks would do the number crunching and come up with a report on critical tasks.

It’s much more convenient now. Tools like Microsoft Project will highlight the critical tasks for you. Celoxis, the tool that we develop, will even alert you in advance if it detects that there is a risk of slipping. More project management tools are available here.

For more information, read about the Critical Path Method on Wikipedia.