COMBINING MULTIPLE MEASUREMENT RESULTS: THE EXAMPLE OF IRMM-083, A 240PU SPIKE WHICH HAS BEEN CERTIFIED AGAINST FOUR OTHER CERTIFIED REFERENCE MATERIALS

During the careful measurement of nuclear samples by destructive analysis methods the proper combination of measured results can raise problems which are not easy to solve. Traditionally, repeated measurements have been combined to yield an average value and a standard deviation, which has then been used as the measure of the uncertainty associated with the mean measured value. The Guide to the Expression of Uncertainty in Measurements (GUM) /1/, however, requires that the uncertainty of a measurement be derived by propagating the uncertainties of the contributions which are combined to make the measurement result. Thus we have the common situation that in a repeated measurement we can calculate the uncertainty of the result value either by applying the GUM methods or as described above by combining the repeated measurements. The extensive measurements described here for the certification of the amount content of a 240Pu solution, IRMM-083, provide an ideal case for examining the conflicting ideas and in particular let the effects of correlations between measurements be quantified on the result of the final certified value.