Manage the Work: The Logic of Critical Path

The actual calculations and logic behind critical path calculations are tedious and time-consuming.  However, from a logical point of view, you can think about it as follows.

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, 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 without causing all the succeeding activities to be late. That's because none of the activities in a logic path 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 as scheduled, or else the entire project will be delayed. 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 established because of the critical path. If there were not a critical path, then there would be at least some float in all the activity paths from start to finish. If there were float everywhere, then why wouldn't you squeeze the float out and finish the project earlier? The answer is that you would. As you moved the end date in 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 the path would have start and end dates that backed up one against the other. There would be no more float on this sequence of activities. This would be the critical path.

For example, say you have a project that is nine months long. After scheduling the work, your project management tool identifies the critical path. Let's assume that there are 22 activities in the critical path, all of various duration's and effort hrs. The second activity on the critical path was estimated to be completed in 8 days. As the project is proceeding, it turns out that this activity actually took nine days to complete. What you will find 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 prior deadline by one day. Unless that extra day can be made up somewhere down the road the project will be completed a day late.

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