Three-Point Estimate Under Pressure
A PM is estimating a data-conversion task. The lead engineer gives a single figure of 10 days based on 'the last time we did something similar.' The PM asks for a three-point estimate. The engineer offers: optimistic 7 days (if the source data is clean), most likely 12 days, pessimistic 25 days (if two legacy formats need manual mapping, which happened on a prior project). The team has 3 engineers available, but only one has done this conversion before. The project also has a fixed 20-day window for this phase. Company policy requires a 15% management reserve on the phase estimate.
1. Compute a PERT expected duration and explain what it tells the PM about the 20-day window.
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PERT expected = (O + 4M + P) / 6 = (7 + 4×12 + 25) / 6 = (7 + 48 + 25) / 6 = 80 / 6 ≈ 13.3 days. That is well inside the 20-day window, but the estimate is asymmetric: the pessimistic tail (25 days) is far from the mean, signalling real risk from the legacy-format scenario. The PM should note that the expected value is not a promise - the distribution has a long right tail, and the 20-day window only covers the mean plus buffer, not the pessimistic case. The single-point 10-day figure understated the task and would have hidden the risk.
2. How should the PM handle the 3-engineer resource and the 15% management reserve?
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Only one engineer has done the conversion, so adding the other two does not scale linearly - Brooks's-law risk applies. The PM should plan the experienced engineer on the critical mapping work and use the others for parallelizable, lower-skill tasks (data profiling, validation), not assume 3× speed. The 15% management reserve applies to the phase estimate: 13.3 × 1.15 ≈ 15.3 days, still inside 20. But the reserve is for management, not the team, and should not be handed out as padding. The PM should keep the reserve visible at phase level, document the legacy-format risk with a trigger, and confirm the 20-day window still holds if the pessimistic case begins to materialise.