Story Points and Velocity

I am re-reading Mike Cohn's book "Agile Estimating and Planning". Chapter four, "Estimating Size with Story Points", introduces the concept of story points and velocity.

Story points represent a relative estimate of the complexity, size or risk in a story. As a relative measure the focus is on assigning stories of similar complexity, size or risk the same number of story points. A relative measure simplifies estimation because it separates size from duration.

Velocity is a measure of the number of story points a team completes in an iteration. Use common sense when managing velocity. For example, if someone takes a day off during the week their contribution to the team's velocty should be reduced.

Velocity also provides the abilty to correct estimates as velocity changes without having to re-estimate the stories themselves. For example, a team with a velocity of 25 might find itself moving closer to 20. This change increases the duration of the project because the total number of story points is known but the rate at which they are completed is reduced.

Duration is derived on Agile projects. It is derived by observing a team's velocity. Duration is the number of story points in a project divided by the team's velocity.

Duration calculations using velocity need to be considered in light of the assumptions velocity brings with it. For example, such calculations assume that all things remain equal over the course of the project. This means that personal changes on a team will impact velocity. It also means that duration calculations should bear in mind the lessons of the cone of uncertainty.