One of the important point in the Scrum Agile project management approach is the fact that user stories are estimated using a relative size and not the “exact” man/days measure used to size effort by traditional approaches. In his blog post, Ilan Goldstein discusses how to explain the relative estimation concept in Scrum.
The relative estimation of user stories in Scrum can be defined in story points that follow a Fibonacci sequence (1, 2, 3, 5, 8, 13, 21, 34, 55, …) or use clothing size (XS, S, M, …). The post explains that the relative estimation applies the principle that is is easier, more accurate and quicker to “compare” than to “break down”. Ilan Goldstein explains also why he prefers using the story points instead of other units like “ideal days”.
Finally, he provides some benefits of relative story point estimation:
1. Rapidly estimation of a long-term product backlog
2. Broader insight from diverse functional experts
3. Ensure that the entire the team understand and assess the merits of the requirements
4. Leverage the knowledge gained from completing legacy work
5. Create some fun during the estimation task with planning poker
Update November 2016: this blog post was originally published on http://www.scrumshortcuts.com/, but is not available anymore