Metrics are the measures that you use evaluate and prioritize your projects.
Project evaluation metrics include characteristics of your projects, estimates of the impacts or consequences that would result from conducting your projects, and characteristics of the external environment that affect the benefits to be derived from your projects.
The most important consideration for selecting metrics is that they provide the information necessary to compute project value. Project evaluation metrics, in other words, are the inputs to your project value model. Also, metrics should be "observables" — quantities that can be monitored so that the actual benefits of funded projects can be measured and verified.