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Why do some teams use numbers from the Fibonacci sequence as story points? Is it just a preference, or is there something more to it?

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6 Answers 6

up vote 34 down vote accepted

Some teams also use powers of two, or have a scale like 1, 2, 5, 8, 20. The idea is that the larger the story is, the more uncertainty there is around it and the less accurate the estimate will be. Using the Fibonacci sequence helps teams to recognise this uncertainty, deliberately creating a lack of precision instead of wasting time trying to produce estimates that might also carry a false degree of confidence.

Dan North's blog post, "The Perils of Estimation", explains this very effectively IMO.

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However it was I believe Mike Cohn who said that they switched from 21 to 20 in their planning poker as 21 seemed "too precise" and 20 looked properly coarse-grained. –  Pawel Brodzinski Jan 4 '12 at 21:11
    
I kind of made that scale up, but it seems sensible - if estimating something you've never done before can ever be said to be sensible, anyway. –  Lunivore Jan 4 '12 at 23:55
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Where did 13 go? –  ashes999 Jan 25 '12 at 20:58
    
In this instance, anything bigger than 8 contains so much uncertainty it might as well be 20. It's just another example of the kind of things people do. –  Lunivore Jan 26 '12 at 8:11

I would add that having the scale non-linear helps with making decisions. It's much easier to say: it's more 8 than 5 than to say it's more 8 than 7.

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So given that the scale for points is non-linear, does it then follow that any velocity derived from said points will not correlate linearly to a planned sprint (sum of points)? Moreover, a velocity of 10 does not mean that planned sprint totalling 10 is logically achievable (but merely likely to be achievable)? –  cottsak Jun 21 '12 at 1:19
    
Seperately, while it's easier to say "8 is more than 5" are the relationships between integers linear? ie. 8 is not just more than 5 but in fact exactly 1.6 times greater than 5? –  cottsak Jun 21 '12 at 1:22
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The relationship is not linear, and that is the point. The relationship is based on orders of magnitude (which is why some teams use powers of 2). –  Andrew Clear Mar 18 '13 at 20:11

Try this simple exercise: What's the difference between stories with consecutive scores like 5 and 6? And that between 3 and 5 or 5 and 8 or 8 and 13 or 20?

The fact that these 'buckets' are further apart imply that you are forced to make a choice between the less/more uncertain stories and choose which bucket is the most appropriate one. The human mind 'sees' a perceptible difference between 5 and 8 story points (or 13 and 20) than it does with 5 and 6 or 10 and 11.

And since the stories are relatively estimated (i.e., 20 is 4 times as much as 5) it's quite difficult to ascertain the 'ratio' of difference between a 5 and 6 than a 5 and 8 (say)

The fact of the matter is this: Increasing the interval between the numbers forces the mind to 'see' a visible difference in magnitude. More so, the variance (standard deviation) would be more pronounced than a linear scale allowing the discussions of differences to be more explicit - i.e., there wouldn't be much difference/discussions if the estimates were 4,5,6 but would be otherwise if they were 3,5,8 (say). The latter has a much higher variance.

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In addition to the answer of Lunivore:

Estimation can be done by using the Fibonacci sequence: 0, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

But the sequence we use most of the time is: 0, 0.5, 1, 2, 3, 5, 8, 13, 20, 40, 100 and ?

0 indicates a user story that doesn't take up any time at all.

0.5 indicates a task that is smaller then the smallest task previously estimated. This often results in changing the previously estimated items to a higher value so the task voted 0.5 gets a value of 1.

20 replaces 21 because the estimation can't be accurate to +/-5%, and 21 seems very precise, whereas a nice round number like 20 seems like what it - an estimate.

bigger than 20 in my opinion, all tasks bigger then 20 should be split up into smaller user stories, as they are too big to be estimated with any accuracy.

? is used when people don't know understand the user story or more info is required

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We almost never exceed 13, and even 8's are considered "large". For us, a 13 is definitely a candidate for splitting up. We strive to keep our stories at 5 and below. –  Bryan Oakley Mar 16 '13 at 4:12

Because the complexity of interactions in a system scales non-linearly with the number functional components.

To a first approximation, let's say you have three components in a system that all interact with each other. That's 3 "interface points" that might cause you to have to change the components to all work correctly with each other. If you have four components, it's not 4 interface points, it's 6, and with 5, it's 10, etc. In general, a complete graph with n nodes (all nodes collaborate directly) has n(n-1)/2 edges, so a system with all components talking to each other has n(n-1)/2 interactions.

1, 2, 3, 5, 8, 13 <- Fibonnacci
1, 2, 3, 5, 10, 15 <- n(n-1)/2 (hypothetical ideal)
1, 2, 4, 6, 16, 32 <- 2^n

As you can see, Fibonnacci tracks, but "falls behind", while powers of two wildly overestimate things with more than 5 components. In the end, Fibonacci is usually chosen because in most systems, n(n-1)/2 over-estimates the number of real interactions in your system: rarely do all components interact with each other directly in a real-world application.

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Trivial, but the 4th term in n^2 sequence should be 8 –  Ed Griebel Jul 8 at 17:05

I think if we put this in a better perspective and we draw the spiral out it shows a better perspective of time spent. So if I go 1 to 2 that is a very small circle in a time frame, but if I go from 1 to 21 and look at the arc it becomes much better to understand in time spent. So although hard to explain with out graphics, take a compass (you remember those) put it on 1 as your point then circle it to your next number or point value and you will get a much better picture of time spent.

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Hey David, welcome to Project Management SE. I suspect you're compass analogy could be valuable, but to make this a more complete answer, you may want to address why teams would use the fibonacci sequence for story points. On our platform, answers get ordered randomly or voted to the top, so it's important they all answer the question for context. For more details on how our site works, please see tour. Good luck and thanks for participating. –  jmort253 Apr 20 at 1:44

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