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Evaluation of Proficiency Testing Results with a Drifting Artifact

Abstract: Proficiency testing (PT) is an evaluation of participant's performance against pre-established criteria by means of interlaboratory comparisons. The normalized error, En, is the most widely used performance statistic for determining the measurement capability of a calibration laboratory. One of the variables in the En equation is Uref, which is the expanded uncertainty of the reference laboratory's assigned value. To evaluate a participant's performance effectively, if any effects of the PT scheme are significant, the additional uncertainties should be combined with the reference laboratory's reported expanded uncertainty to estimate Uref. Among such uncertainties, the stability of artifacts is an important uncertainty component in the PT scheme, especially for a calibration laboratory. Based on practical PT experience, most artifacts can be regarded as sufficiently stable if the difference between three reference laboratory measurements is small. In such cases, the median of the three measurements is usually chosen as the assigned value, and its reported expanded uncertainty is used as the Uref value. However, some artifacts, such as standard resistors, drift over time. This leads to questions about how to accurately determine the assigned values and expanded uncertainties of these artifacts. This paper presents a PT scheme for standard resistors that demonstrates the evaluation of PT results with a drifting artifact.