What is a disadvantage of using PERT/CPM?

Using PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method) involves certain prerequisites and assumptions about project activities. The correct answer highlights that project activities must be clearly defined and stable for these methodologies to work effectively.

In PERT/CPM, the nature of project planning relies heavily on defining each task in terms of specific start and finish times, as well as clear dependencies between tasks. If tasks are not well-defined or are subject to frequent changes, it can lead to complications in project scheduling and management. This is because the calculations and the network diagrams used in both methods assume a certain level of stability and clarity which, if absent, can cause inaccuracies in time estimates and mismanagement of resources.

The emphasis on stability is crucial; if project requirements or tasks evolve frequently during the project life cycle, it complicates the scheduling process and may render the PERT/CPM model ineffective. Thus, the necessity for well-defined and stable project activities is indeed a disadvantage of using these methodologies.