Wednesday, June 17, 2009

Why estimate using Story points?

The other day I was talking about why bothering using Story Points instead of just estimating everything in hours. Here's a link to the slides.

Traditionally, the aim has been to try to estimate time by looking intensely at a problem (or a task, activity, requirement,…) – so traditionally there’s a lot of effort spent on breakdown, analysis & estimation. But at the same time, we all know that estimating time is very hard and often pretty inaccurate – because software development is research-oriented and there are things that surface only once we start digging in, which then change the picture. We all know this – yet those time estimates tend to become commitments down the line.

I argue that it’s not only the problem (task, activity, requirement,…) that is hard to foretell; the environment where we solve the problem also has a significant effect on the amount of time it will take to complete. And often the environment is much harder to foretell than the problem itself. So, in the end it doesn’t matter how long time you spend analyzing, you will still have a large unknown in the shape of the environment.

The point I want to make is that estimating in Story Points is about recognizing that Time is a result of the Size of a problem solved within a certain Context.
Time = Size x Context

Story Points is a unit for representing Size. This way Context becomes something we can grasp by comparing Size to Time – which is why we measure Velocity.


  1. The last is not the least. If we improve, for example, the way we work, our estimation in time will reflect that. The estimation of size should still be the same. In the former case there will be no measured increase of velocity, as in the latter.

    So by measuring size, we can see that the change in how we work, was worth the while.

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