Performance measurement is about tracking and understanding relative progress, in order to make more effective progress in the future. Links and correlations are often established by looking at trends over time, e.g. when policies, outputs or resources changed, or by establishing comparison groups, e.g. using disaggregation or international benchmarks. It is common to spend as much time setting-up comparators as producing measures.
Quantitative vs. qualitative
Quantitative measurement of every aspect of an agency's function is not feasible, nor is it necessary. Many outcome, impact and output measures are qualitative. Whether measures are qualitative or quantitative, they must give users a useful indication of what has been achieved, and be comparable to other measures.