How does duration estimation differ between PERT and CPM?

How does duration estimation differ between PERT and CPM?

• A. PERT uses fixed deterministic durations while CPM uses probabilistic estimates.
• B. CPM uses probabilistic estimates for durations.
• C. PERT uses probabilistic time estimates (optimistic, most likely, pessimistic), while CPM uses fixed deterministic durations.
• D. Both use fixed durations with no uncertainty.

Answer: (C)

The key idea is that PERT models uncertainty in how long activities will take, while CPM treats durations as fixed numbers. In PERT, you don’t rely on a single duration. Each activity has three estimates: optimistic, most likely, and pessimistic. Those feed into a probability-based approach to compute an expected duration (often E = (O + 4M + P)/6) and a sense of spread (the uncertainty in that activity’s time). This lets you work with a probabilistic project completion time and assess risk. In CPM, durations are deterministic: each activity has one fixed value, and you compute the schedule from those numbers to find a single critical path and a single project duration with no built-in uncertainty. So, the correct description is that PERT uses probabilistic time estimates (optimistic, most likely, pessimistic), while CPM uses fixed deterministic durations. The other statements misstate where uncertainty appears or imply both methods treat time the same way.

The key idea is that PERT models uncertainty in how long activities will take, while CPM treats durations as fixed numbers. In PERT, you don’t rely on a single duration. Each activity has three estimates: optimistic, most likely, and pessimistic. Those feed into a probability-based approach to compute an expected duration (often E = (O + 4M + P)/6) and a sense of spread (the uncertainty in that activity’s time). This lets you work with a probabilistic project completion time and assess risk.

In CPM, durations are deterministic: each activity has one fixed value, and you compute the schedule from those numbers to find a single critical path and a single project duration with no built-in uncertainty.

So, the correct description is that PERT uses probabilistic time estimates (optimistic, most likely, pessimistic), while CPM uses fixed deterministic durations. The other statements misstate where uncertainty appears or imply both methods treat time the same way.