I. Introduction
The purpose of a measurement is to gain information about the value of a quantity. However, due to measurement uncertainty, the information obtained is incomplete in the sense that the exact value of the quantity remains unknown. The measurement uncertainty comprises not only the random variability of repeated observations under apparently identical conditions due to unpredictable or stochastic temporal or spatial variations of influence quantities but also unknown systematic effects due, for instance, to the error of indication of the measuring instruments and to incomplete corrections for recognized effects from influence quantities. In this sense, incomplete information due to random or unknown systematic effects in the measurement is treated on an equal basis, i.e., each quantity influencing the measurement is treated as a random variable to which a probability distribution can be assigned [3].