Proficiency tests with uncertainty information: analysis using the maximum likelihood method
In this study, we report the application of the maximum likelihood method to the analysis of proficiency test data when uncertainty information is given and a reference laboratory does not exist. There are two causes that could impair the quality of analysis using the maximum likelihood method: the existence of an unknown random effect, and outliers. The conditions under which performance evaluations can be appropriately conducted are discussed in this study. To avoid serious impacts from these two causes, the maximum permissible standard uncertainty of an unknown random effect and the minimum permissible standard uncertainty of the values reported by a participating laboratory are quantified. Through simulations, the maximum and the minimum permissible standard uncertainties are found to be 0.3 and 0.5 times the intermediate magnitude of the standard uncertainty that the participants are expected to report. We believe that our proposed procedure based on these criteria is sufficiently simple to be employed in actual proficiency tests.
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