activity slack

The determination of the critical path of a project requires forward and backward calculation steps (see “Scheduling projects: How to determine the critical path using activity slack calculations?”) and allows for the calculation of three different slack values. In this article, the following three definitions of activity slack will be discussed:

• Total slack
• Safety slack
• Free slack

The Critical Path Method (CPM) is a project scheduling technique to analyze and represent the tasks involved in completing a given project. It incorporates a trade-off between an activity’s duration and cost and relies on concepts similar to the program evaluation and review technique (PERT, see “The Program Evaluation and Review Technique (PERT): Incorporating activity time variability in a project schedule”).

The Program Evaluation and Review Technique (PERT) is a project scheduling technique to analyze and represent the tasks involved in completing a given project. It incorporates activity duration variability and relies on similar concepts as the critical path method (CPM, see “The Critical Path Method (CPM): Incorporating activity time/cost trade-offs in a project schedule”).

The critical path is a series of project activities with a zero slack value connected by precedence relations (see “Scheduling projects: How to determine the critical path using activity slack calculations?”). It contains all activities that are critical for the project duration and results in the generally accepted critical path rule: