Any proposed quality metric must be suitable for use. A suitable metric must have four attributes.
- It must be well-defined, so everyone has the same understanding of its meaning.
- It must have a range, a meaningful domain, and a known slope (that is, the shape of the curve is understood).
- It must expressed in raw numbers, not by percentages. (This is so that successive measurements can be combined.)
- It must be reproducible.
In addition, a proposed metric should correspond to something important, such as a CSF or a process milestone. Finally, a proposed metric should be easily obtained or well worth the effort to obtain.
A simple example of a metric is the number of spelling errors per 1000 words. The range is 0 (no errors) to 1 (every word is misspelled); the domain is the entire range; the slope is negative (larger numbers are worse than smaller numbers). The metric is objective and, given a specified wordlist authority, repeatable.