When first starting out in Agile, a lot of developers (and especially management) naturally try to find a relationship between story points and hours or days. Story points are an estimate of how long something will take, but it is abstracted as a relative relationship. Although there is a relationship between story points and hours or days, that relationship varies by scrum team. Trying to get your team to go through the paradigm shift can be challenging, especially for project managers who are trying to transition to Scrum. This challenge comes down to the difference between accuracy and precision. In case you're not sure, here's a quick example to help illustrate the difference:
For the first part of this example, take a medium-size cup from the break room or any fast food restaurant. Hold it up for all to see, then set it on the table. Do not let them touch the cup. Tell them, "Without discussing with your peers, write down exactly how tall the cup is in inches."
Next, ask them to write down the length of the circumference (around the edge) of the lip of the cup, also in inches. As they are writing down these numbers, note how long it takes them to come up with a value.
Once everyone is done, ask each member for the values they wrote down. Write each one on a whiteboard (or use sticky notes). For fun, ask why there are so many different values and whose estimate they would be willing to commit to.
For the next part, ask them to repeat the exercise; except this time they must use relative estimation. To mix things up, pull out a can of Coke (or a different size cup).
Ask the team, "Relatively speaking, how much larger is the circumference of the lip of the can to the height of the can?" Give them four choices:
A. About the same size
B. About 1.5 times larger
C. About twice as large
D. About 4 times larger
Note: It doesn't matter how tall the can or cup is for them to give an estimate. Also, notice how much faster they can complete the exercise and how much more they are willing to commit to an answer.
I like this example because it forces people to think about similar things (length) in an unusual way (height vs. circumference). Trying to visualize and calculate the circumference is difficult for many people to do in their head. This is similar to the difficulty team members go through when trying to compare two different stories.
To precisely estimate the amount of time it takes to design, code, and test some functionality of an application is complicated, often wrong, and can be a large waste of time. This is because people in general tend to be better at relative sizing, as opposed to absolute sizing. Being able to use relative sizing is one of the biggest advantages of using story points over time (hours or days).
In conclusion, we are accurate when we say the circumference of the lip of cup is about two times larger than the height (and the circumference of the lip of the can is about 1.5 times larger than the height); however, we are not precise. Similarly, we are accurate when we size Story A as a 2, or two-and-a-half times smaller than Story B (a size 5). We don't need to know how long it will take to do Story A or Story B. We just need to know that Story B is about two-and-a-half times more (time, risk, effort, and complexity) than Story A.
Additional comments and images for this article can be found at http://scrummasterfrank.wordpress.com/2011/10/13/accuracy-vs-precision-in-story-point-sizing/
Frank Rios is an Agile Coach and Certified Scrum Professional (CSP) with over 15 years of experience in IT. His combination of business and technical expertise helps companies by increasing communication between business and technical teams, reducing ambiguous requirements, and decreasing time-to-market.
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