Measurements are always inaccurate because a measured value always represents only an approximation to the expected value ("true value") of a physical quantity. It is impossible to make a single perfect measurement. Thus, when specifying measured values, there must always be an indication of the uncertainty of the measurement. Thus, the old-fashioned term of measurement error has given way to the new term of measurement deviation, because measurement uncertainty, from this point of view, is not necessarily due to error, but to various influences on the measurement process, the measured object or the measuring instrument. Both systematic and random deviations can contribute to measurement uncertainty. For measurements of high accuracy, such as calibration measurements, systematic deviations are usually insignificant. The determination of measurement uncertainties and thus also the evaluation of the quality of measurement data is carried out according to the standards of the GUM (Guide to the Expression of Uncertainty in Measurement) and the ISO (International Organization for Standardization). The procedures defined therein as an international agreement are used for the determination and presentation of measurement uncertainties taking into account any influencing variables.