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Type A Evaluation of Measurement Uncertainty: Frequentist or Bayesian?

The Guide to the Expression of Uncertainty in Measurement (GUM) recommends using the so-called Type A method to calculate the standard uncertainty associated with the best estimate of a quantity measured under repeatability conditions. Instead, for quantities about which other forms of information are available, the GUM says that Type B methods of evaluation should be used. The former method is based on the frequentist interpretation of probability, while the latter involves essentially the Bayesian theory. This mixing of concepts is not convenient, as it leads to the GUM being inconsistent. For this (and other) reasons, a draft for a major revision of the GUM was prepared. Perhaps the greatest modification introduced in this draft with respect to the GUM had to do precisely with a new (Bayesian) method to obtain the uncertainty of a quantity for which more than three repeated indications are available. Had this draft been accepted, the revised GUM would have become consistent. In the present article, the fundamentals of the current and proposed Type A evaluation methods are reviewed. A brief explanation of why the proposed draft was not accepted is given also.