Scheduling Fundamentals
Critical Path Method (CPM)
The Critical Path Method is the scheduling technique that calculates each activity's earliest and latest possible dates from durations and logic, and identifies the longest chain of dependent activities that determines the project's finish date.
Definition
CPM works in two passes over the network of activities and relationships. The forward pass moves from the project start, computing each activity's early start and early finish from its predecessors. The backward pass moves from the project finish, computing each activity's late start and late finish from its successors. The difference between late and early dates is total float; activities with the least float form the critical path. Because every date in a CPM schedule is a calculation, not an entry, changing a duration, a relationship, or a calendar changes the dates downstream -- which is exactly what makes CPM schedules usable as the basis for delay analysis.
Example
A three-activity chain of 5, 10 and 3 working days with Finish-to-Start logic has a critical path of 18 working days; a parallel 4-day activity attached to the same start and finish has 14 days of total float.