The essential technique for using CPM is to establish a model of the project that includes a list of all activities required to complete the
D: Duration ES: Eariy start EF: Early finish LS: Latest start LF: Latest finish
project, the time duration that each activity will take to completion, and the dependencies between the activities.
By using these values, CPM calculates the longest path of planned activities to the end of the project and the earliest and latest time that each activity can start and finish without making the project take longer time.
This process determines which activities are "critical," which are the activities on the longest path, and which have "total float," as these activities can be delayed without making the project longer.
Any delay of an activity on the critical path directly impacts the planned project completion date (i. e., there is no float on the critical path). A project can have several critical paths. An additional parallel path through the network with the total durations shorter than the critical path is called a sub-critical or non-critical path.