The Critical Path Method (Calculate Schedule) is a modeling process that defines all the project's critical activities which must be completed on time.
The Calc tool bar button on the Gantt and PERT (found in most GUI-based PM software) windows calculates the start and finish dates of activities in the project in two passes.
The first pass calculates early start and finish dates from the earliest start date forward.
The second pass calculates the late start and finish activities from the latest finish date backwards.
The difference between the pairs of start and finish dates for each task is the float or slack time for the task (see FLOAT).
Slack is the amount of time a task can be delayed without delaying the project completion date.
A great advantage of this method is the fine-tuning that can be done to accelerate the project.
Shorten various critical path activities, then check the schedule to see how it is affected by the changes.
By experimenting in this manner, the optimal project schedule can be determined.