Why might time estimates in PERT/CPM be unreliable?

Time estimates in PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method) can be deemed unreliable primarily because managers might subjectively adjust the estimates. While PERT/CPM methodologies rely on a probabilistic approach to project scheduling—taking into account optimistic, pessimistic, and most likely durations—these estimates are often influenced by personal judgment and experience.

When managers tweak these estimates based on their subjective views, they may inadvertently introduce bias, leading to either overly optimistic or overly pessimistic timelines. This subjectivity can skew the accuracy of project scheduling, as individual assumptions can vary widely depending on varying experiences or perspectives on potential risks and challenges. Therefore, the reliability of time estimates hinges not only on the method used but also on the individual's perception and interpretation of data and experiences related to the project.