29th June 2011.
Critical path refers to the sequence of activities that must be completed on time for the entire project to be completed on time. 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 schedule flexibility and where you do not. For example, you may have a whole series of activities that end up running late, yet the overall project will still complete on time if the late activities are off the critical path. Similarly, if your project is falling behind, placing additional resources on activities that are off the critical path will not result in the overall project completing any earlier.
Note that the critical path will only be calculated if you have sequenced all of the activities in your schedule. The critical path relies on an understanding of the successors and predecessors of each activity. If your activities are not sequenced, the critical path may be calculated erroneously.
The Logic Behind 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. 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 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).
The project end-date is what it is because of the critical path. For example, say you have a project that is nine months long. Your project management scheduling 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 eight 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.
Why is the Critical Path Important?
Frankly, on many projects it is not necessary to determine the critical path. This is especially true for small projects. However, for larger and more complex projects, it is important to understand the critical path. If the project is trending late and if you are trying to proactively get back on schedule, 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 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 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.
The Critical Path May Change
There are many sequences of activities on a project to get from the beginning to the end. There may, in fact, be multiple critical paths, if they all have no float and all lead to the same end-date. Usually if there are multiple critical paths, they overlap for many of their activities
Given that there are many, many paths through the schedule, it’s possible for the critical path to change. For instance, say you have the same example as above, with 22 activities over nine months. Let’s assume that there is another path of work that includes 19 activities and takes 8 ½ months. If you tried to accelerate the schedule to complete the project in eight months, it gets a little complicated. First you would want to focus on accelerating the activities in the nine–month critical path. However, once the critical path is reduced to 8 ½ months, this second critical path emerges that has the same overall timeline. Compressing the original critical path further will not make the project end earlier, because this second path is still going to take 8 ½ months to complete. In this case, both paths must be accelerated (or perhaps some activities that are common to both can be accelerated).
The other way the path may change is if activities off the critical path get delayed. In the example above, let’s say that one of the activities on the 8 ½ month path ends up taking an extra three 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 one week late. |
