Chapter 9. Fit Criteria

in which we show how measuring a requirement makes it unambiguous, understandable, and, importantly, testable

Fit, as we use the term here, means a solution completely satisfies the requirement. That is, the solution does exactly what the requirement says it must do or has the property the requirement says it must have, no more and no less. But to test whether the solution fits the requirement, the requirement itself must be measurable. As a simple example, if the requirement calls for a length of rope "of a suitable size," it is obviously impossible to test any delivered solution. By contrast, if the requirement says the rope shall be "2 centimeters in diameter and 2 meters long," then it becomes a simple matter to test whether the delivered solution fits the requirement.

"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind."

Source: Lord Kelvin

Of course, attaching a measurement to a length of rope is easy. Attaching a measurement to some requirements is much more difficult, but still possible, and absolutely necessary.

The measurement of the requirement is the fit criterion. It quantifies the behavior, the performance, or some other quality of the requirement.

So far in this book we have mainly dealt with the description of the requirement. The description states the stakeholder's intention for the requirement, and is the normal thing stakeholders say when they are giving you requirements. But to know precisely what they intend, you must quantify the description. Once you measure the requirementthat is, express it using numbersthere is very little room for misunderstanding.

Beck, Kent, and Cynthia Andres. Extreme Programming Explained: Embrace Change (second edition). Addison-Wesley, 2004.