The five examples in this paper present a varied and interesting collection of contexts in which models and methods for assessing measurement uncertainty are used. Consistent with Dr. Possolo's assertion that 'probability distributions are well suited to express measurement uncertainty', his paper presents and advocates the use of probability models to guide simulations whose variability expresses the uncertainty connected with a measurement.Actually, although this is not always clear in Dr. Possolo's discussion, there is more than one kind of uncertainty involved with measurement. We must differentiate between pre-data uncertainty and post-data uncertainty. I say more about this distinction in Section 2 of this discussion, and this distinction serves as a guiding theme for my comments on Dr. Possolo's examples (and the methodology used in these examples) in Section 3. Section 4 concludes with some remarks about the question of how best to communicate information about uncertainty of measurement.
Pre-data uncertainty and post-data uncertaintyPre-data uncertainty is concerned with the variability about the true value of the measurand of the values produced by a particular method of measurement-it is uncertainty about the accuracy of the measurement methodology. In statistics, indices of variability of point estimators or coverage probabilities of confidence intervals are used in the frequentist paradigm of inference to select procedures that will give the most accurate measurement, resulting in minimal uncertainty. Distributions of the point estimators provide not only these indices but also more detailed insight into the performance of the measurement method. Examples of such an approach in Dr. Possolo's paper are the arsenic-in-oyster-tissue example of Section 3 and the viscosity example of Section 4. In the arsenic-in-oyster-tissue example, the parametric bootstrap is used to estimate the distribution of the maximum likelihood estimator of the consensus value under the assumption that has the value O attained by the maximum likelihood estimator. One could instead simulate the exact distribution of the estimator when a particular value of the measurand holds. If the value specified for the measurand is correct, the distribution will be the correct pre-data uncertainty of measurement. The bootstrap distribution thus estimates the correct pre-data distribution of the measurement rather than giving it exactly. There is an unfortunate tendency in the literature to regard this bootstrap distribution as a post-data expression of uncertainty because the data are involved in choosing which distribution to simulate, but this is not the case-what is obtained is an estimate of the correct pre-data distribution of values for the measurement.When expressed in terms of distributions, post-data uncertainty commonly takes the form of a distribution for the measurand (not the measurement) conditional upon observed measurement values. To obtain such a distribution, we start with a prior distribution for the measurand that express...