Why you should use Fibonacci Numbers to Assign Story Points

I have been asked many times why not to use linear sequence. The reason not to use linear numbers I that linear scales do not capture the complexity and uncertainty involved in developing user stories. The complexity is captured by using a Fibonacci sequence such as: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89.

It is hard to perceive a difference between stories with consecutive scores like 5 and 6 or 7 and 8. The human mind perceives a difference between 5 and 8 story points or 13 and 21 story points. The fact that Fibonacci values are further apart implies that we are forced to make a choice between the amount of significant effort a task will take to complete. 

Moreover, it is difficult to establish a ratio between a 5 and 6 than 5 and 8 or 8 and 13. The fact that the intervals between values is increasing forces the mind to perceive a visible difference in magnitude as well.

While estimating work using story points, a point value is assigned to each item. The values of story point are insignificant; the most significant part of story points is the relative ratio of values. A story with 2 story points should require twice as the work effort as a story with one story point. A story with 2 story points should also require two-thirds the work effort as compared to a story with 3 story points. The calculation of effort of work involved in completing an item from product backlog is nonlinear.

To find a valid sequence to use for Story Points, the characteristics a sequence need to have includes base values, consistent ratios, and the sequence needs to be nonlinear.