WHAT IS A METRIC?

There are two kinds of metrics:

A metric is a measurable attribute of an entity. For example, project effort is a measure (that is, a metric) of project size . To calculate this metric you would need to sum all the time-sheet bookings for the project.
A primitive metric is an item of raw data that is used to calculate a metric. In the preceding example, the time-sheet bookings are the primitive metrics. A primitive metric is typically a metric that exists in a database but is not interpreted in isolation.

Each metric comprises one or more collected metrics. Consequently, each primitive metric must be clearly identified, and its collection procedure must be defined.

Metrics to support change or achievement goals are often first-derivative over time (or iterations or project). We are interested in a trend and not in the absolute value. If our goal is "Improve quality," we must check that the residual level of known defects diminishes over time.