The critical path is "critical" because it drives the end date. 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.
I attended an end-of-project meeting a few years ago. The project was relatively successful, but the participants were talking about things they could have done better. The client stated that more time should have been spent in training. The project manager replied, "I agree. Next time I will place training on the critical path." A little later the project manager noted that she should have placed client approvals on the critical path so they were completed sooner.
At that point I had to jump in, since the term "critical path" was not understood correctly. "Critical path" refers to the sequence of activities that must be completed on schedule for the entire project to be completed on schedule. In fact, the activities on the critical path may not be very important at all. The critical path may include very mundane activities. The critical path is "critical" because it drives the end date. 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.
A layman's description of critical path
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." By definition, if an activity has flexibility, or float, associated with its start and end date, then it is not on the critical path.
Let's look at those activities where you do not have the flexibility in the start and end dates. These activities can't be completed earlier because they're waiting for the completion of another activity. They also can't 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 from project start to end that must be started and completed exactly as scheduled. 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).
A layman’s description of critical path II
The project end date is what it is because of the critical path. If there were not a critical path, there would be at least some float in all the activity paths from the start of a project to the finish. If there were float on all paths, 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 at least one path would have start and end dates that backed up against each other. There would be no more float on this sequence of activities. This would be the critical path.
Why is it important to understand the critical path? It is important because you can't effectively manage your workplan without understanding the critical path. If your project is running late, you need to know the critical path since that is the only way to accelerate the timeline and get back on schedule.
Want to know more? Read more details on how to manage the schedule using critical path in next week’s column.