Project Management Tips and Techniques
Explaining the Importance of the Critical Path
Tom Mochal

Critical path refers to the sequence of activities that must be completed on schedule for the entire project to be completed on schedule. If the end date for the project has slipped, it is because at least one activity on the critical path did not complete on time. It is important to understand the critical path sequence to know where you have flexibility and where you do not. For instance, you may have a whole series of activities that end up running late, yet the overall project will still complete on time. On the other hand, if your project is falling behind, placing additional resources on non-critical activities will not result in the overall project completing earlier.

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 for the project. This flexibility between the earliest time an activity CAN be completed and the latest time when it MUST be completed is called float. There is similar float if the activity has flexibility between the earliest time it CAN start and the latest time it MUST start. By definition, if an activity has flexibility, or float, associated with its start and end date, then it is NOT on the critical path. 

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 than scheduled without causing all the succeeding activities to be late. That's because none of the activities that follow have any flexibility, or float, in their start and end date. All of these activities back up tightly against other activities that precede or succeed them. The critical path consists of the longest sequence of activities that must be started and completed as scheduled or the entire project will be delayed. In other words, it is the longest sequence of activities with zero float. If any activity on the critical path is late, the entire project will be late (unless the time can be made up somewhere else on the critical path). 

The project end date is what it is because of the critical path. If there were not a critical path, then there would be at least some float in all the activity paths from start to finish. If there were float everywhere, you could squeeze the float out and finish the project earlier. As you moved the end date to finish earlier, you would start to remove some of the float. However, at some point, the float would be gone from one of the paths. This would be a point where each activity on the path would have start and end dates that backed up one against the other. There would be no more float on this sequence of activities. This would be the critical path.

For an example, say you have a project that is nine months long. After scheduling the work, your project management tool identifies the critical path. Let's assume that there are 22 activities in the critical path, all of various durations and effort hours. The second activity on the critical path was estimated to be completed in 8 days. As the project is proceeding, it turns out that this activity actually took nine days to complete. What you will discover is that now the entire project will take nine months and one day. Delaying the completion of the second activity by one day made the schedule for the entire project go over its deadline by one day. Unless that extra day can be made up somewhere later in the critical path, the project will be completed a day late. 
Frankly, on many projects it is not necessary to determine the critical path. This is especially true for small projects. However, if you are trying to accelerate the overall schedule (in other words you are trying to complete the project earlier than currently scheduled) it is very important to identify these critical path activities. Remember that the critical path is where the activities are backed up one against the other with no float. Therefore, unless you are able to accelerate activities on the critical path, the end date for the entire project will remain the same. Applying additional resources to activities that are not on the critical path may allow those activities to be completed early, but they will not effect the overall project end date.

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