Last week I explained that "critical path" refers to the sequence of activities from project start to end that bump up back-to-back against each other without any float. This sequence of activities 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 to know where you have flexibility and where you do not. You may have a whole series of activities that end up running late, yet the overall project will still complete on time since 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.
Pretend you are the project manager for these examples:
Q1. One of the activities that you estimated to take two weeks has ended up taking five weeks instead. Should you panic?
A1. Is the activity on the critical path? If it is, your project will be delayed three weeks and you should look for ways to get back on schedule. If the activity is not on the critical path, then the activity delay may be a problem (for instance a budget problem), but may not cause a delay in the project.
Q2. Your sponsor just told you that the project needs to be completed three weeks early. You have a plan to assign a second person to complete a six-week activity in three weeks. Should you pat yourself on the back for a job well done?
A2. Is the six-week activity on the critical path? If it is, you may be able to meet the earlier end date. If the six-week activity is not on the critical path, you are just kidding yourself.
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 estimated end date relies on your ability to identify and shorten the critical path.
The critical path may change
There are many sequences of activities (paths) on a project to get from the beginning to the end. Given that there are many, many paths through the workplan, it's possible for the critical path to change. Remember that the critical path has zero float. However, other paths may have only a small amount of float. If a second path, for instance, has three days of float, what will happen if one of the activities on the second path took an extra five days to complete? All the sudden, the second path would become the critical path, and the original critical path would now have two days of float.
In summary, if you need to manage the duration of your project, you need to understand the critical path. The only way to impact the overall completion date is to impact activities on the critical path.