Abstract
Key comparison (KC) measurements are the backbone of validating the calibration and measurement capability (CMC) of national metrology institutes (NMIs) against each other. Recommendations providing procedures for the preparation and execution of KCs as well as supplementary comparisons are given. They are justified by mathematical reasoning and by aspects of practical use. Arguments and a formal description of calculating the key comparison reference value (KCRV) by its weighted mean based on the expanded measurement uncertainty expressed by a confidence level of 95 % will be presented. Moreover, it will be explained why the calculation of the normalized error relates to the weighted mean only and why its absolute value for the expanded measurement uncertainty shall be below 1 to pass the equivalence criterion (En) of comparability. A deeper discussion is given on how reference values and measurement results shall be treated if few individual measurement results differ significantly from the others. Finally, it will be explained how the proposed style of the graphical presentation of measurement results allows a clear visual interpretation of the comparison. Besides the formal description, numerical examples and diagrams are presented.
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