Story Point Estimates Are Best Thought of As Ranges

Consider the scenario of needing to store 10 liters of water. Two buckets are available: one with an 8-liter capacity and another with a 13-liter capacity.

Which bucket would you store the water in?

Clearly, the 13-liter bucket. Ten liters of water don’t fit in an eight-liter bucket. In fact, you’d use the 13-liter bucket for all amounts of water from nine through 13.

And so it is with the values you choose to use when estimating stories. Each value you use should be thought of as a bucket. And that estimate value is used for all stories between that value and the next lower value.

For example, when playing Planning Poker many teams will use a modified FIbonacci sequence of 1, 2, 3, 5, 8, 13, 20, 40 and 100. The 13-point card should be used for any story the team estimates larger than eight and no larger than 13.

That is, each story point value is implicitly a range--just like a bucket can hold a range of amounts of water.

To speed up Planning Poker, you want the team to think about these buckets.

The team’s job is not to put an estimate on a product backlog item. The job is to put the story in the right bucket. Yes, I know, that in both cases, someone grabs a pen and writes a number on a story card.

But tell the team to picture a set of buckets in front of them labeled 1, 2, 3, 5, 8, 13 and so on (or whatever sequence it uses). And to picture themselves throwing story cards into the buckets.

This helps team members move away from the feeling that estimates need to be perfect and precise. They don’t. Product backlog items merely need to be "thrown" into the right bucket.