0, 1, 2, 3, 5, 8, 13… The sequence that conquered both mathematics and sprint retrospectives.
Quick refresher: what is Fibonacci?
The Fibonacci sequence is a series of numbers where each value is the sum of the two before it: 0, 1, 2, 3, 5, 8, 13, 21, 34…
In agile, we use a simplified version for estimates: 1, 2, 3, 5, 8, 13, 21 (sometimes with 0, 1/2 and infinity as extras).
Why these values and not 1, 2, 3, 4, 5?
Two main reasons:
1. The gaps grow with complexity. Distinguishing a 7 from an 8 is illusory. Nobody can really say whether something is 12.5% more complex than something else. Distinguishing an 8 from a 13, on the other hand, is meaningful: it's almost twice as big.
2. Uncertainty grows with size. The bigger a ticket, the less precise you can be. The Fibonacci sequence reflects this reality: large tickets have wide intervals, small ones have fine intervals.
The psychological effect
Studies have shown that people tend to anchor their estimates on the available values. If you offer 1 to 10, everyone hesitates between 4, 5 and 6. With Fibonacci, the choices are sharper: it's a 5 or an 8, not a 6.5.
In practice, a ticket estimated at 21 or more is often an alarm signal. It deserves to be split before entering a sprint.
And the 0? And infinity?
- 0: the ticket is already done or takes less than an hour. No need to estimate it seriously.
- Infinity (or ?): the team doesn't have enough information to estimate. Clarification is needed first.
