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  • Fibonacci numbers for agile estimates

    Posted on February 18th, 2011 Thibaut 142 comments
    Sprint Planning

    During the sprint planning, team members give estimates about the tasks they’re going work on in the coming sprint. Maybe some of you have already experienced that endless discussion about “is this a 10h or a 11h task ?”. This is where the planning poker, based on the Fibonacci numbers comes to the rescue.

    Fibonacci numbers are an integer sequence defined by the following recurrence relation :

    Fn = Fn-1 + Fn-2

    Which produces the following sequence :

    0, 1, 1, 2, 3, 5, 8, 13, …

    See the point ? Fibonacci numbers are particularly relevant for giving estimates. While one can easily estimate a 1h or 2h task, I don’t think a lot of people (if any) can tell for sure that a task will take 11h and not 12h. But with this technique, deciding between 8 and 13 is much easier. Some pokers also contain the 1/2 value, but the most useful addition is surely the “?” card, meaning that an accurate estimate isn’t possible because of the lack of visibility and more information should be given by the product owner.

    Planning poker

    When hesitating too much about two consecutive values, consider the time dedicated to testing (manual testing, unit tests, …) and don’t forget that velocity isn’t really compatible with quality ;)