Peter C. Webb

A simple test is proposed for assessing whether reference material values are fit for purpose according to the uncertainties associated with these values. The approach involves comparing individual reference value uncertainties ( u RM ) with the corresponding target uncertainty ( u H ) calculated from a modified form of the Horwitz function, u H = 0.01 c 0.8495 , where c is the mass fraction of the reference value. Providing u RM < u H /3, the uncertainty contribution from the reference value will cause an increase in the combined uncertainty no greater than 5.4% and the reference value can be considered fit for purpose in the calibration and validation of respective measurement results. In applying this methodology, it is convenient to calculate a quality factor (QF = 3 u RM / u H ) for each reference value, which would then be considered fit for purpose when QF < 1. Examples are shown for reference materials characterised by different methodologies (NIST certification, the IAG certification protocol, GeoReM preferred values and the Geo PT programme). Calculation of the median quality factor from all quoted reference values in individual reference materials allows an overall assessment to be made of the quality of the material and the effectiveness of the methodology used for its characterisation.