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). This scale was popularized by Mountain Goat Software as the default for Planning Poker.
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.
