Statistical reassessment of calibration and measurement capabilities based on key comparison results

According to the CIPM mutual recognition arrangement, a calibration and measurement capability (CMC) uncertainty for a laboratory offering a particular calibration or measurement service is normally expressed at a 95 % level of confidence. When laboratories' CMC claims are unsupported by the relevant key comparison (KC), after publishing the final report of the KC, modified values may have to be assigned to their CMC uncertainties. In many cases when CMCs apply to a continuous interval of values such as mass fraction or wavelength, KCs are carried out for selected discrete values of the quantity concerned. Since the comparison at each discrete value may only support the CMC uncertainty at that point, it is not immediately apparent how to modify the CMC uncertainties. Under realistic assumptions, we developed a method that is applicable in such an instance and for which the reported CMC uncertainties are amplified so that they are underpinned by the results of the KC. The amplification factors depend on the laboratories' degrees of equivalence (DoEs) for these discrete values, judiciously adjusted to achieve consistency with the key comparison reference values. The method is based on the patterns in the individual behaviour of the DoEs of the participating laboratories for the discrete values, implying the presence of correlation associated with the DoE values. It applies when the weighted mean of some or all of the measured values reported by the participating laboratories in the KC is used to obtain the KC reference value.