The Meaning of Fibonacci Sequence in Agile User Stories

The Fibonacci sequence is a serie of numbers in which each number is the sum of the previous two preceding numbers, starting with 0 and 1. The standard Fibonacci sequence is 0, 1, 2, 3, 5, 8, 13, 21, 34, 55 and 89.

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It was invented by an Italian mathematician Leonardo Fibonacci (or Leonardo of Pisa), also simply known as “Fibonacci” in the Middle Ages, and included in his book Liber Abaci (book of calculation), published in 1202. This book on arithmetic, written in Latin, introduced Hindu-Arabic numeral system in Europe and was one of the first Western books using symbols that were similar to the modern Arabic numerals.

However, it was only later during the 19^th century that this sequence of numbers was rediscovered and widespreadly used in fields like mathematics and biology.

In Agile teams it is used to estimate in a realistic way – how long it takes to complete a task/user story in a sprint (planning) – using story points. So, story points represent the size, complexity and effort needed through a number from the Fibonacci sequence. This means that the higher the number, the more complex and the more effort (due to more risk and uncertainty) it will be needed to complete the story point.

Usually, 21 is used as the highest number of the sequence in Agile context and a high estimate means that we need to break the task down into smaller ones.

The following table, shared in the article of Julee Everett, presents a kind of synthesis of practical Fibonacci sequence starting in 0 and ending in 21. For Everett, “For example, 1 Story Point means that (…) the item can be delivered quickly, perhaps in one day or less. On the other hand, an item assigned 13 Story Points means it is very complex and could take multiple weeks to complete. (…)”

In fact, the use of Fibonacci sequence in Agile estimating works as a starting scale for comparing items, from a relative sizing perspective. Although, humans are bad when it comes to estimate something, as remembered by Everett, they are good when it comes to compare things (relative sizing).

Additionally, we can always customize the Fibonacci sequence and there are other options to the standard version of the sequence, such as: modified Fibonacci sequence (0,0.5,1,2,3,5,8,13,20,40,100); powers of two (1,2,4,8,16,32,64); simplified scales, in which are included for example, the t-shirt sizes (S, M, L, XL) and the five fingers (1,2,3,4,5).

Some Agile teams combine still the use of Fibonacci sequence with an estimation technique involving cards, called planning poker. But…How it works?

“In Planning Poker, each team member has a modified deck of cards that corresponds with the Fibonacci sequence. When a task is up for estimation, each member individually picks a card that best matches their estimate for that item. Everyone then reveals their cards simultaneously and discusses their estimates until they reach a consensus.” 

Finally, and in what respects to the advantages of using the Fibonacci Sequence in Agile context, they are, according to Kelechi Udoagwu , the following:

1.Collaboration in cross-functional teams; 2. A scale, that is easily understandable, for comparing user story points; 3. More accuracy of estimates in project planning; 4. More team participation and engagement because of the involvement of the whole team, with each one presenting his/her own perspective.