Why 8 plus 8 is 13 in Story Points?

Recently during a talk with one of the stakeholders I said, that from my experience 13 Story Points tasks are usually split into two 8 Story Points tasks. The stakeholder instantly laughed and suggested to exchange money with me in a similar way. He proposed that he will give me $13 and I will have to give him only two times $8 in return.

I need to add a small disclaimer here, that not always 8 plus 8 is 13, sometimes 5 plus 5 is also 13 and even 13 plus 5 can still be 13. It depends.

Back to the story. Why such an equation is true when it comes to Story Points and is not when we speak about money?

Because Story Points measure time right?

Not exactly. Or more precisely – not only. Story points are units of measuring the overall effort needed to fulfill the definition of done. During estimation, all the things like complexity, uncertainty and time required need to be taken into account.

Single backlog item estimated in Story Points can be directly converted to neither time nor money. However, when we look at a large group of tasks, we can calculate the average cost of the implementation or average time needed for the release.

So why when we split the task into half, Story Points doesn’t follow?

Reasons might be different. For instance, some amount of the preparation is required for both A and B, so we need to calculate this work in both A and B. Other possibility is that there is high uncertainty hidden in both of them. Another option is that the technology used in A and in B is unknown for the team. Either way – estimates might differ from the original half.

Is there any reference to the real world?

It is the real world case! But if you are still not convinced that Story Points are subject to different limitations than numbers, then I propose you to remember a very simple metaphor.

  1. Think about the effort needed to fulfill a story, as about a pile of sand.
  2. Treat Story Points as buckets with defined capacity.

We all know that we can fit a little more sand in a bucket than we can fit water. And sometimes… you can fit 16 liters of sand to 13 liters bucket.

What do you guys think? Do you have your own answer to similar questions?

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