One of the most difficult tasks related to quantitative analysis is to obtain reliable
input data. There are several possible sources for this data. For existing systems or
organisations, measurement can be one of the most reliable methods, although it is
not easy to do this in a correct way: among others, it should be clearly defined what
exactly is to be measured, the number of measurements must be sufficient, and the
measurements must be taken under various circumstances that can occur in
practice.