Estimating the uncertainty of stability for matrix CRMs

The new version of ISO Guide 34 requires producers of certified reference m aterials (CRMs) to include contributions of possible instability to the ove rall CRM uncertainty, to obtain a value for the uncertainty in compliance w ith the Guide to the Expression of the Uncertainty in Measurement (GUM). A pragmatic approach to estimating the uncertainty of stability is presented. It relies on regression analysis of stability data with subsequent testing of the slope of the regression line for significance, If the slope is foun d to be statistically insignificant, a shelf life is chosen and the uncerta inty connected with this time is estimated via the standard deviation of th e slope. This uncertainty is included in tile overall uncertainty of the CR M, This approach is explained with examples showing its applicability to ma trix CRMs.