There is no such thing as “absolute estimation”

Posted on August 25th, 2015 in Agile by siddharta || 3 Comments

estimates

One of the discussion topics I’ve seen raised again and again in agile circles is about absolute vs relative estimation. The theory goes that human beings find it difficult to estimate in absolute numbers like estimating the height of a building in metres, but find it easy to do relative estimations like estimating that a coconut is four times as big as a guava. And this is the reason why estimating a story in story points (relative estimate) is superior to estimating a story in hours (supposedly an absolute estimate). This theory finds it’s way into every agile discussion on estimation, as well as many trainings.

And yet, this theory, is completely wrong! (At this point I’ll admit that I’ve myself said this before)

There is no such thing as “absolute estimation”

Let us first get one thing out of the way: There is no such thing as absolute estimation.

ALL common measurements-like measuring the height of a building for example-is a relative measurement. After all, a metre is just a standard baseline defined by the International Bureau of Weights and Measures. If you call something as 5 metres long, it just means that it is 5 times as big as this standard baseline. Someone else might say that it is 16 feet long, and they get a different number because they use a different baseline. So measuring distance in feet or metres is just a relative estimate of multiples to a reference value.

It is no different from setting a reference story as 1 story point and measuring multiples against that reference.

The same goes for other types of measurements, like time for instance. Someone just decided to divide a day into 24 pieces and defined a reference value of 1 hour. When we say something will take 5 hours, we just mean it is 5 times as long as this reference. It is a relative estimate, just like story points.

So it is completely wrong to say that story pointing is better because humans are better at relative estimating, because estimating in hours is ALSO a form of relative estimating.

Comparing vs Estimating

One aspect of the first statement is true: it IS easier to say that a coconut is four times as big as a guava, than it is to estimate a building height in metres. The reason is not anything to do with relative or absolute estimating (remember that BOTH are relative estimates), but with how well humans can compare things. We are good at comparing objects that are similar in measure, but bad at comparing things that are of much bigger or smaller scale. As an example, it is easy to use a guava as a reference and say that the coconut is about four times bigger. But if I were to ask how big the Earth is relative to a guava, then most would be wildly off the mark (by wildly I mean more than a trillion times off the mark). However, if I were to ask how big the Earth is relative to Mars, folks would be reasonably close to the answer.

Putting this to use

Alright, that’s a lot of theory, but is it of any use? Fortunately it is!

Here is the anti-pattern: I’ve seen teams define a tiny 1 point story as a reference baseline and then try to size larger features or epics relative to this tiny story. It just doesn’t work because we are no good at sizing a big thing using a tiny thing as a reference. And then the team wonders why the superior “relative estimation” is not working.

If you’ve read this far, the fix should be obvious: Create a reference story at every scale. You should be having a reference 1 point story for tiny fixes. A reference 10 point story (13 points if your team rigidly sticks to fibonacci) for new features. A reference 100 point story for larger epics, and so on. Then compare your stories against the appropriate reference story.

So, to get back to the hours vs story point debate, does this mean that if we could create reference stories in hours, like saying that this is a 1 hour story, that one is a 10 hour story and so on, and then comparing stories against the reference, then an hourly scale would work just as well as story points? I’ll leave that question open to address in the next blog post.

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