Once we know the tasks that are on the critical path we can consider various options for shortening the duration of that path. Those options would include “crashing” the project by adding resources to critical path tasks so they finish sooner. As you are adding resources to the critical path, you need to periodically check to see if the critical path has changed. As we add resources to the tasks on a given path, the duration of that path decreases. Once another path has the longest duration it becomes the critical path. Adding more resources to any tasks not on the critical path will not affect the duration of the project.
When Do We Use Critical Path
We use the critical path when we need to shorten the duration of the project. The most common situation is when the initial draft of the schedule takes longer than the sponsor wants.
Why Do We Use the Critical Path
We use the critical path technique because it identifies where to add resources to reduce the project duration. Adding resources to the critical path tasks, (the longest path in the project) is much cheaper than adding resources to all of the tasks. The reason is that the other paths have slack; they can finish later than scheduled without delaying the project completion. It’s always useful to identify the critical path as you near the end of your project scheduling. By knowing the critical path and how much slack the other tasks in the project have, you can identify opportunities for shortening the duration.
How Do We Use the Critical Path
Let’s examine a project with three paths. Each task in the path is identified with an alphabetic letter so the path ABDE starts with task A, then goes to B, then to D and ends with E.
Duration |
ABDE |
ACFE |
ADGE |
Path duration |
10 days |
14 Days |
13 Days |
Slack |
4 Days |
0 Days |
1 Day |
We see that path ACFE has the longest duration, which is 14 days. We also see that path ACFE has no slack. When the path has no slack, the tasks on it cannot finish later than the baseline schedule without increasing the duration of the project. When they have slack, they can increase their duration until they use up all of their slack. At that point, that path becomes the critical path.
Critical Path Scenario
Chris Pimbock was a new project manager in his organization and he was preparing the schedule to present to the project sponsor. The sponsor had been very clear during the initial planning session that the project had to be finished within 45 days. When Chris checked the first draft of the schedule, the duration of the project was 50 days. Chris calculated the duration of each of the four paths through the project and found that the critical path was indeed 50 days long. However, the other three paths were all less than 40 days. Chris then examined the five tasks that were on the critical path and found that one task, the engineering work, had 20-day duration. That told Chris that the easiest place to shorten the duration by 5 days was the task that had 20-day duration. Chris looked at the resources assigned to that longest task on the critical path and found that one engineer had been assigned to do 160 hours of work. Chris contacted the engineering department and asked if a 2nd engineer might be available to help on the task. In discussing options with the manager, Chris also specified that if they added a 2nd engineer both engineers could be finished within 10 days rather than the original 20 day schedule. The engineering department manager agreed to supply a 2nd engineer to the project and Chris rescheduled the whole project. The duration was now 40 days and that allowed Chris to present a schedule, which had 5 days of slack. That meant that the critical path tasks could slip by 5 days and the project would still finish on time.
Deep Dive on This Topic with Additional Articles:
Using the Critical Path to Optimize Your Project Schedule
