For the critical path to be calculated, you must complete the following prerequisites:
The critical path calculation uses the schedule dates, project calendar information, constraints and constraint dates (for the Finish no later than constraint type), and dependencies, to determine the following four parameters for each activity:
Earliest start date (ES): the earliest time at which the activity can start considering the constraints and dependencies for the activity. The earliest start date can change as the project progresses and changes are made to the planned schedule.
Earliest finish date (EF): equal to the earliest start time for the activity plus the time required to complete the activity. The earliest finish date can change as the project progresses and changes are made to the planned schedule.
Latest finish date (LF): the latest time at which the activity can be completed without delaying the project, and considering the constraints and dependencies for the activity.
Latest start date (LS): equal to the latest finish time minus the time required to complete the activity.
The float or slack is the amount of time that an activity can be delayed past its earliest start or earliest finish without delaying the project finish date. The float for an activity is the time between its earliest and latest start time, or between its earliest and latest finish time.
If the float or slack is zero, the task or activity is considered as critical and is highlighted in the Gantt chart view. The critical path is the path through the project WBS in which none of the activities have float, that is, the path for which ES=LS and EF=LF for all the activities in the path. Any delay in the activities in the critical path delays the project. To accelerate a delayed project, reduce the total time required for the activities in the critical path.
During project exection, the critical path can change because inordinate delays in activities outside the critical path may begin to affect the project finish date, and cause those activities to be on the critical path.