A project is a complex system, so one metric will generally not be sufficient. Too many metrics are also undesirable; important information will be lost in the jumble. Strive to define a minimum set of project metrics to give a balanced project view.
Select metrics for:
Objectivity: If metrics are evaluated by different people, all will get the same result.
Accessibility: Good metrics are easy to collect.
Clarity: Verify through discussion that everyone understands the measurement process and uses consistent units of measure.
Frequency: Collect data frequently enough to support the results you desire, but not so often that it represents unduly high overhead.
Importance: Collect only metrics that will make a meaningful difference; ...