Duration

What it counts

Duration is the number of working days from a task's start to its finish. It is a different number from effort, the total labor poured in. A task two people work on for three days each carries 6 person-days of effort but a duration of 3 days. Conversely, two days of effort from someone who can spare only an hour a day can stretch to a duration of two weeks. Duration measures length on the calendar; effort measures people's time — two different axes.

Why it matters

Duration is the elementary input of schedule computation. The Critical Path Method adds duration to the start date over the working-day calendar to produce the finish, and that finish becomes the successor's start. One wrong duration shifts the dates of the entire chain behind it. Whether duration counts working days or calendar days also changes the finish of the same "10 days" — with weekends and holidays in between, 10 working days spans more than two calendar weeks. Without team agreement on the basis, every reader of the schedule imagines different dates.

Common misconceptions

The oldest one: adding people shrinks duration proportionally. Effort can be divided, but review, handover, and communication costs grow, so duration does not shrink in proportion. Milestone duration confuses people too — a milestone is a point in time, not work, so its duration is 0. Finally, remember that a summary task's duration is not the sum of its children's durations but the span from the earliest start to the latest finish.