One of the concepts new scrum teams struggle with is how to relate story points and hours. People want an easy answer, such as “one story point = 8.3 hours.” The truth is, though, that the relationship, while real, is not quite that easy to quantify and will vary greatly from team to team.
Suppose for some reason you track how long every one-story-point story takes a given team to develop. If you did, you’d likely notice that while the elapsed time to complete each story varied, the amount of time spent on one-point stories takes on the shape of the familiar normal distribution.
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Now suppose you had also tracked the amount of time spent on two-point user stories. Ideally, the two-point stories would be two times the mean time of a one-point story and will follow a similar distribution curve as shown in this image.
This will never be exactly the case, of course, but a team that does a good job of estimating will be sufficiently close for reliable plans to be made from their estimates.
The relationship between points and hours is a distribution. One point equals a distribution with a mean of x and some standard deviation. The same is true, as well, for two-point stories, and so on. While there might be some overlap in elapsed time between 1 and 2 point stories (some one-point stories might turn out to be bigger than the team thought; some two-point stories end up being smaller), or between 2 and 3-point stories, there will rarely be any overlap between a 1-point story and, say, a 13-point story in terms of actual elapsed time.
With all that being said, I caution you against formally relating points to hours. You run the risk of forgetting that each team’s mean time to completion will be as different as their idea of what constitutes a point. Points are, after all, a relative measure.
Mike Cohn is a co-founder of the Scrum Alliance and is the author of three popular books on agile and Scrum. He can be reached through www.mountaingoatsoftware.com.
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