La influencia de la homogeneidad en la evaluación de la estabilidad de los materiales de referencia

Accurate estimation of the uncertainty of stability is crucial for certified reference materials (CRMs), as it forms a component of the overall uncertainty budget. In case of between-unit heterogeneity, results obtained on the same unit of a CRM are not independent which leads to a bias in the estimated uncertainty of stability when used ordinary least squares. This bias depends on the ratio of between-unit heterogeneity to method repeatability and the number of replicate analyses performed per unit and can reach nearly 50 % for as few as three replicates per unit. A study on actual certified values shows that this bias can influence the certified values. The study shows that employing linear mixed-effects models (LME) with restricted maximum likelihood (REML) for parameter estimation provides consistent estimate of uncertainty of stability, even in the presence of significant between-unit heterogeneity. Simpler, alternative approaches for specific scenarios, such as well-balanced datasets are also provided.