The ‘Critical Path’ is a sequence of connected tasks without any slack time (or ‘float’) between them. These tasks form the longest consecutive sequence of tasks in the project and thus determine the finish date of the project. Due to the lack of any slack time between tasks in the critical path, any deviation in one task causes a deviation in the project as a whole. Therefore, the project manager must monitor the critical path continuously, as well as other project paths with slack times that shrink gradually possibly creating a new, longer critical path.
The connections between various tasks together with the constraints on each task generate a sequence that is similar in its essence to a domino chain.
If the distance between two dominos is shorter than the length of the dominos, then dropping the first domino will cause the second domino to fall. The first domino has no free slack, which is what causes the domino following it to fall, i.e., causes a delay in the task following it, too.
If the distance (float) between the second and third dominos is sufficiently large, the second domino can fall without causing the third domino—which represents the end of the project—to fall.
The second domino has enough free slack to avoid disturbing the next one. The first domino, by virtue of being placed before the second domino, has total slack that prevents it from causing a delay in the project as a whole.
It should be clear that only a task whose total slack is zero or less than zero is on the critical path, whereas a task whose total slack is larger than zero is not on the critical path.
The critical path of the project provides the project manager and other parties involved in the project with a crucial, managerial focal point. Therefore, the critical path must be controlled and the results communicated on a regular basis. When a task that lies on the critical path or a task that feeds the critical path is late, the project’s work focus should be to close up the gap and return the project to its planned timing as soon as possible. Many times a project will run late being faced with negative slack times. In such cases, focus on the tasks that are furthest behind and continuously attempt to shrink these gaps.
The critical path also provides the key points for shortening the duration of the project. By virtue of being the critical path, it determines the duration of the project. As a result, this is where shortcuts can be introduced. Note that introducing a shortcut into a task that is not along the critical path is worthless; doing so will simply increase the available float in the project but will not affect its overall completion date.
Remember that it is very likely that taking less time to complete the project will result in an increase in its cost. Therefore, it is important to control the connection between the duration of the project and its cost so that the budgetary framework of the project is maintained.