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Experimental Design and Data Evaluation Considerations for Comparisons of Higher-Order Value-Assigned Materials

Published

Author(s)

David L. Duewer, Hugo H. Gasca-Aragon, Katrice A. Lippa, Blaza Toman

Abstract

The analysis of higher-order value-assigned materials (HOVAMs) such as certified reference materials (CRMs) and value-assigned proficiency testing materials can help assess the equivalence of chemical measurement processes. When two or more such materials are available for a given measurand then confidently establishing the equivalence of measurement processes requires that those materials are known to be capable of yielding equivalent results, particularly if those materials were value-assigned by different organizations. Evaluating the degrees of equivalence among materials that differ in analyte quantity and perhaps matrix composition requires an approach other than that used to assess results for samples of a single material. We have more than a decade of experience with an approach that compares the assigned values of a group of materials that deliver different levels of the same measurand, much of it at the highest metrological level under the auspices of the Consultative Committee for Amount of Substance Metrology in Chemistry (CCQM). This approach compares the assigned values of the materials to a simple linear model of the relationship between those values and measurement results made under repeatability conditions. In addition to accessing the metrological equivalence of specific materials, the equivalence of the value-assignment capabilities of the organizations that certified the materials can also be accessed. Rather than focusing on specific comparisons, this report summarizes our experience with the design of and analysis of such comparisons and provides numeric and graphical tools for estimating degrees of equivalence. We divide the required tasks into four steps: 1) design, 2) measurement, 3) definition of a reference function, and 4) estimation of degrees of equivalence. We regard the experimental design and measurement tasks as most critical to the eventual utility of the comparison, since creative mathematics can not fully compensate fo
Citation
Accreditation and Quality Assurance

Keywords

CCQM (Consultative Committee for Amount of Substance – Metrology in Chemistry), GDR (Generalized Distance Regression), HOVAM (Higher-Order Value-Assigned Material), Metrological Comparability, Metrological Equivalence, Reference Function (RF)
Created August 22, 2012, Updated November 10, 2018