At the Iteration level, User Stories are the basic unit of commitment by the implementation team. The relative size of User Stories is measured using “Story Points”. As a general rule, people are usually good at estimating that story “A” is twice as big as story “B”. When things go beyond an order of magnitude, the estimates begin to get vague – for example, is story “A” 53 times larger than “B” or 62 times larger?
By measuring past performance and estimating the number of story points the team successfully completes each iteration it is possible to identify a set of user stories that the team can commit to delivering in each iteration. Teams commit to only one iteration at a time, although they may plan 2-3 iterations ahead. At the end of each iteration the success of the team is measured, and future iterations are adjusted accordingly. This way the team does not make commitments that it can’t keep, and the business can ensure that the team is always working on the highest value/priority deliverables.
The story point estimation model tends to be self-correcting over 3-6 iterations as the team begins to understand how many story points they can actually deliver each iteration. They also get better at estimating the size of stories based on their past performance and similar stories implemented in previous iterations.
NOTE – User stories are committed to and owned by the entire team. Everyone on the team is jointly responsible for the success or failure of the team to deliver the completed user story.