The critical path method is a schedule network analysis technique that is performed using the schedule model. The critical path method calculates the theoretical early start and finish dates, and late start and finish dates, for all schedule activities without regard for any resource limitations, by performing a forward pass analysis and a backward pass analysis through the project schedule network paths.
The resulting early and late start and finish dates are not necessarily the project schedule; rather, they indicate the time periods within which the scheduled activity should be scheduled, given activity durations, logical relationships, leads, lags, and other known constraints.
Calculated early start and finish dates, and late start and finish dates, may or may not be the same on any network path since the total float, which provides schedule flexibility, may be positive, negative, or zero. On any network path, the schedule flexibility is measured by the positive difference between early and late dates, and is termed “total float.”
Critical paths have either a zero or negative total float, and schedule activities on a critical path are called “critical activities.” Adjustments to activity durations, logical relationships, leads and lags, or other schedule constraints may be necessary to produce network paths with a zero or positive total float. Once the total float for a network path is zero or positive, then the free float — the amount of time that a schedule activity can be delayed without delaying the early start date of any immediate successor activity within the network path — can also be determined.