Estimating effort
Effort, duration and deadline, the person-day, the estimation methods (analogy, experts, three points, parametric) and the traps that derail a schedule: Brooks's law, optimism bias.
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Effort of a task: what is it?
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The amount of work needed to complete it, independent of the calendar and of the number of people, expressed in person-days.
Sixty person-days is sixty days for one person or, if the work splits well, twenty days for three: the effort stays the same, the calendar does not.
A project manager writes: “the task is 20 person-days, so it takes 20 days”. What is wrong?
- It mixes up effort and duration: duration also depends on staffing
- It forgets that a person-day is counted in calendar hours
- It forgets that the client sets the effort, not the project team
- There is no mistake: an effort in person-days is a duration in days
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It mixes up effort and duration: duration also depends on staffing
Effort measures work, duration measures elapsed time. Two full-time people would do 20 person-days in 10 days, one half-time person in 40.
… leads people to estimate from the best-case scenario; one remedy is to compare past estimates with the … durations.
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Optimism bias leads people to estimate from the best-case scenario; one remedy is to compare past estimates with the actual durations.
People usually underestimate their own tasks: the record of past gaps offers a correction based on facts, not on good intentions.
Duration of a task: what is it?
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The calendar time elapsed between its start and its end, which depends on the effort, the staff assigned and their availability.
An effort of 10 person-days given to one person working half-time lasts 20 working days: the duration is derived from the effort, it is not the same thing.