Often a reference material is certified based on data from more than one measurement method. This situation occurs when no single method can provide the necessary level of accuracy and/or when there is no single method whose sources of uncertainty are well understood and quantified. Measurements from different sources, different laboratories, different instruments, and different methods can exhibit significant between-method variability, as well as distinct within-method variances. A common goal in the analysis of such data is to compute a best consensus value and to attach a meaningful consensus uncertainty to that value. The results of the combined XRD and optical microscopy data sets are presented in Table 6. Three methods for combining these data sets are presented and discussed below.