Uncertainty calculations in the certification of reference materials 3. Stability study

To serve as a measurement standard, a (certified) reference material must b e stable. For this purpose, the material should undergo stability testing a fter it has been prepared. This paper looks at the statistical aspects of s tability testing. Essentially, these studies can be described with analysis of variance statistics, including variant regression analysis. The latter is used in practice for both trend analysis and for the development of expr essions for extrapolations. Extrapolation of stability data is briefly touc hed upon, as far as the combined standard uncertainty of the reference mate rial is concerned. There are different options to validate the extrapolatio ns made from initial stability studies, and some of them might influence th e uncertainty of the reference material and/or the shelf-life. The latter i s the more commonly observed consequence of what is called 'stability monit oring'.