Five Important Things to Know About Critical Path

Some people think the critical path is where the critical work is performed. Similarly, it was just a few weeks ago that a project management stated that she needed to pick the "right" critical path for her project. Both of these definitions are incorrect.

Critical path refers to the longest path through the schedule and represents the shortest time it takes to complete the project. The work on the critical path might or might not all be "critical". The critical path is determined by the work and the dependencies of your schedule. You do not pick the "right" critical path.

The critical path is an important aspect of your project schedule. Here are five things to know about critical path.

1. Float refers to schedule flexibility

On every project, no matter how complicated, there are always some activities that can be started earlier or completed later without jeopardizing the final completion date. This flexibility between the earliest time an activity CAN be completed and the latest time when it MUST be completed is called "float".

2. The critical path has no float

Now let’s look at those activities where you do not have the flexibility in the start and end-dates. These activities cannot be completed earlier because they are pending the completion of another activity. They also cannot be completed later without causing all the succeeding activities to be late. All of these activities back up tightly against other activities that precede or succeed them. In other words, the critical path has zero float.

3. The critical path is the longest path

The various network paths in your schedule have various lengths. The longest path is the critical path. Since it is the longest path, every other path will, at some point, have to wait while the critical path work is completed. This "wait" time is the float.

4. You need to understand critical path to manage the project with precision

If the project is trending late it is very important to identify the critical path activities. Unless you are able to accelerate activities on the critical path, the end-date for the entire project will not change. Applying additional resources to activities that are not on the critical path will not affect the overall project end-date. Your chance to make an impact on the projected end-date relies on your ability to identify and shorten the critical path.

5. The Critical Path May Change

Given that there are many, many paths through the schedule, it’s possible for the critical path to change. For instance, say you have a project with a critical path (longest path) of 22 activities over nine months. Let’s assume that there is another path of work that has 19 activities and takes 8 ½ months. There are two weeks of float on this path. Let's say one of the activities on the 8 ½ month path ends up taking an extra four weeks. Because there was only two weeks of float in the path, it will now become the critical path and force the entire project to complete in 9 1/2 months.

You will not be able to calculate critical path unless you are sequencing all activities. However, all project managers should understand this concept - even if you are not able to actually calculate one for your project.