This article is the first of a series of that will talk about Agile estimation and planning. There is a great deal of information available in books and online describing Agile estimation techniques. This series of articles attempts to summarize some of the available information based on our practical experience and make it readily assessable to our colleagues and clients so that they can better understand our estimation approach. This first article addresses the concept of relative estimation. Subsequent articles in this series will address the effort vs. time, team estimation, and how we can use the combination of relative estimation and team estimation to improve the accuracy of our estimates.

As a trainer I know that people are much more impacted by an experience than an explanation. Unfortunately it’s difficult to provide an experience through a blog entry, so I’ll have to use an explanation in the form of an example from a recent class.

One of the starting points for the Agile estimation approach is the recognition that *people are poor at estimating in terms of absolute values*. To illustrate this point in class I’ll often use a partially filled bottle of water and ask the class to estimate the remaining volume in milliliters (or fluid ounces if you prefer imperial measurements). I typically do this by asking everyone to take a post-it note an write their estimate down, not sharing with others. After everyone has written down their estimate I’ll collect the post-it notes and post them on a wall or whiteboard, reading off the values and arranging them with the highest value at the top and the lowest value at the bottom. This is always a fun exercise because the numbers inevitably demonstrate great divergence. For example in a recent class the low estimate was 100ml and the high estimate was 500ml. Five orders of magnitude in difference. While this simple exercise exaggerates the concept, it consistently demonstrates *people are poor at estimating in terms of absolute values.*

So if we are poor at estimating in absolute values what is the alternative?

The alternative is to estimate in relative terms.

When training I demonstrate this by continuing the exercise started above. Again, I ask the participants to write their estimates on a post-it note, but this time instead of asking them to estimate the milliliters (or fluid ounces), I ask them to estimate the amount of water remaining in the bottle as a percentage value in comparison to the capacity of the bottle. Using information from the same class noted above, the participants offered estimates that ranged from 60% to 80%; still a significant gap, but no longer orders of magnitude in difference.

So the first step we take in improving our estimation using an Agile approach is to use relative instead of absolute values for our estimates. Our relative value is the estimated effort that implementing a given feature will take.

In the second part of this series we’ll look at this topic in more detail to further understand why relative effort is our preferred approach.

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