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This document explains concepts and provides approaches to the following aspects of the production of reference materials:
— the assessment of homogeneity;
— the assessment of stability and the management of the risks associated with possible stability issues related to the properties of interest;
— the characterization and value assignment of properties of a reference material;
— the evaluation of uncertainty for certified values;
— the establishment of the metrological traceability of certified property values.
The guidance given supports the implementation of ISO 17034. Other approaches may also be used as long as the requirements of ISO 17034 are fulfilled.
Brief guidance on the need for commutability assessment (6.11) is given in this document, but no technical details are provided. A brief introduction for the characterization of qualitative properties (9.6 to 9.10) is provided together with brief guidance on sampling such materials for homogeneity
tests (Clause 7). However, statistical methods for the assessment of the homogeneity and stability of reference materials for qualitative properties are not covered. This document is also not applicable to multivariate quantities, such as spectral data.
Note ISO 33406 gives more information on the production of reference materials with one or more qualitative property values.

1.1 This document gives guidance on the production of RMs having one or more assigned qualitative property values. Qualitative properties, also called categorical properties, can be nominal or ordinal. A nominal property has values that partition the set of materials that have it into classes such that all the materials in the same class have the same value of the property, and the only comparison that can be made between values of the property is of whether they are identical or different. An ordinal property is similar, except that the comparisons that can be made between two values of the property are of relative rank order; that is, whether one is lower, equal to, or higher than the other. This document focuses on nominal properties.
1.2 This document gives specific guidance on the production of RMs having one or more assigned qualitative property values, including ways to express related uncertainties and to establish traceability.
NOTE The concepts of traceability and uncertainty address characteristics similar to those addressed by the concepts of traceability and measurement uncertainty as used in the metrology of quantitative properties.
NOTE RMs of pure substances for which a quantitative purity value is assigned should be prepared in accordance with the recommendations for quantitative properties in ISO Guide 35[2].
1.3 This document supplements ISO 17034 (General requirements for the competence of reference material producers)[7] and related ISO Guides such as ISO Guide 35 (Reference materials —Guidance
for characterization and assessment of homogeneity and stability).[2] While ISO Guide 35 provides some guidance on the production of RMs with qualitative property values, Guide 35 and ISO 17034 primarily focus on the production of RMs with quantitative property values. ISO 33406 therefore does not describe aspects related to the production of RMs with quantitative property values.
NOTE Annex A provides examples of types of reference materials within the scope of this document.