Does the use of Fibonacci numbers in planning poker affect effort estimates?

Background: The estimation technique Planning Poker is common in agile software development. The cards used to propose an estimate in Planning Poker do not include all numbers, but for example only the numbers 0, ½, 1, 2, 3, 5, 8, 13, 20, 40 and 100. We denote this, somewhat inaccurately, a Fibonacci scale in this paper. In spite of the widespread use of the Fibonacci scale in agile estimation, we do not know much about how this scale influences the estimation process. Aim: Better understanding of the effect of going from a linear scale to a Fibonacci scale in effort estimation. Method: We conducted two empirical studies. In the first study, we gave computer science students the same estimation task. Half of the students estimated the task using the Fibonacci scale and the other half a linear scale. The second study included four estimation teams, each composed of four software professionals, estimating the effort to complete the same ten tasks. Two of the teams estimated the first five tasks using the Fibonacci scale and the last five using the linear scale. The two other teams used the scales in the opposite sequence. Results: We found a median decrease in the effort estimates of 60% (first study) and 26% (second study) when using a Fibonacci scale instead of the traditional linear scale. The scale difference in the effort estimates decreased as the developers' skill increased. Conclusion: The use of a Fibonacci scale, and possibly other non-linear scales, is likely to affect the effort estimates towards lower values compared to linear scales. A possible explanation for this scale-induced effect is that people tend to be biased towards toward the middle of the provided scale, especially when the uncertainty is substantial. The middle value is likely to be perceived as lower for the Fibonacci than for the line