Any proposed quality metric must be suitable for use. A suitable metric must have four attributes.

  1. It must be well-defined, so everyone has the same understanding of its meaning.
  2. It must have a range, a meaningful domain, and a known slope (that is, the shape of the curve is understood).
  3. It must expressed in raw numbers, not by percentages. (This is so that successive measurements can be combined.)
  4. 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.