Agile estimating and planning succeed because estimates of size and duration are separated. We start by estimating the size of a project’s user stories in story points. Because story points are abstract and notional, they are pure estimates of size. We then estimate a rate of progress, which we call velocity. Our estimates of size and velocity are then combined to arrive at an estimate of duration. Our estimating and planning process benefits from this clear and total separation of estimates of size and duration.
