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Are there good arguments for using the modified Fibonacci series (0, 1, 2, 3, 5, 8, 13, 20, 40, etc) instead of a geometric progression (1, 2, 4, 8, 16, 32, etc) when estimating story points in Scrum (or any agile methodology, really)?

I know that Scrum does not specify Fibonacci, or any specific system, but it is definitely the most popular.

I also see that there are good reasons the either of these systems would be better than a linear progression - increasing uncertainty, removing time consuming - and meaningless - arguments (is this a 6 or a 7?).

So is it just by convention and history that Fibonacci is (almost) all that is mentioned when talking about story point scales or are there specific arguments for it over geometric?

Scott Miller
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    I'm voting to close this question as off-topic because [project management is now off-topic on Stack Overflow](//meta.stackoverflow.com/questions/343829/is-stack-overflow-an-appropriate-website-to-ask-about-project-management-issues/343841#343841). Ask these questions on [SoftwareEngineering.SE](//softwareengineering.stackexchange.com/) and [ProjectManagement.SE](//pm.stackexchange.com/) instead. (Unfortunately, this question is too old to be migrated.) – robinCTS Oct 29 '17 at 17:17

2 Answers2

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First, you're thinking of a "geometric progression", not a "binomial progression" (which is not a real thing).

As for which.... it doesn't matter very much. The Fibonacci series actually approaches a geometric progression as the numbers get larger, so clearly the two have a lot in common. The baseline story units matter far more than the set you choose.

Sneftel
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This is what I believe is beneficial if you use Fibonacci:

1- You don't have to compare complexity relative to other stories that precise. If you are not using Fibonacci series, you may end up comparing which story is bigger twice or 4 times relative to another story, the idea is to have user stories with the lower points. So if you are getting to the higher point range, we don't want to have focus on them and decide if it is 4 times bigger than the user story assigned just now.

2- Fibonacci numbers can be found in many natural patterns, so it might be more natural to us estimating the user stories by them. https://en.wikipedia.org/wiki/Fibonacci_number

3- Back to the lower point stories, see the difference between 8 and 13 and 8 and 16. There is no in-between points. The whole point is to give flexibility when it comes to the stories you want to accomplish within an sprint (which should be fairly simple, so sticking to 2 3 5 8 13 would be way better than 2 4 8 16 32)

  • (BTW the Fibonacci sequence has a 21 instead of 20, usually they simplify that and make it 20)
  • (If I have to argue only by one, I would chose the 3rd one)
sheidaei
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