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ISO 33406:2024 - Approaches for the production of reference materials with qualitative properties
In 2015, ISO/TC 334 (formerly ISO/REMCO) published ISO/TR 79, which summarized the state of the art of the production of reference materials (RMs) with qualitative properties. The lack of an international vocabulary for terms and definitions for qualitative properties added an extra challenge in 2015 and ISO/TR 79 did not define or limit the use of the term "qualitative property". "Qualitative property" was therefore used as superordinate for nominal and ordinal properties. "Categorical properties" is a synonym for "qualitative properties". ISO/TR 79 lists examples of RMs that either have a certified value for a qualitative property or can be considered as in-house RMs characterized for a qualitative property. The examples listed are based on the principles elaborated in ISO 33405 and ISO Guide 80, but ISO/TR 79 did not undergo a consensus-building process.
For RMs, nominal properties are a particular kind of qualitative property (the other kind are ordinal properties). The identity of a polychlorinated biphenyl (PCB) and the species of a maple tree (genus Acer) are nominal properties where the values of the properties are the particular chemical species name or biological species name (e.g. PCB 105 and Acer saccharinum, respectively). The former has more than 200 possible values, the latter more than 160. The only meaningful comparison between values of a nominal property is whether they are identical or different.
Ordinal properties have values that can be ordered (i.e. ranked) from smallest to largest or from lowest to highest, but for which neither differences nor ratios are meaningful, even when their values are represented numerically.
EXAMPLE 1
Stage, as defined by the American Joint Committee on Cancer, is a property of solid tumour cancers (e.g. breast, colon or lung), whose possible values are the Roman numerals I, II, III and IV. However, one is not entitled to say either that stage IV is two times “worse” than stage II or that the difference in severity between stages III and I is the same as the difference in severity between stages IV and II.
EXAMPLE 2
The Mohs hardness of a mineral is expressed relative to a scale ranging from 1 (for talc) to 10 (for diamond) and can include half-integer values (e.g. 5 ½ for enstatite). However, fluorite (4) is not two times harder than gypsum (2), nor is the difference in hardness between topaz (8) and apatite (5) the same as the difference in hardness between quartz (7) and fluorite (4).
There are no unambiguous rules for expressing qualitative properties in various fields such as chemistry and biology. The following examples illustrate the confusion that exists. Qualitative properties can be described in different ways depending on the individual property or even on the way in which a specific property is expressed. For example, "colour" can be seen as a qualitative property of which "red" is a value. Alternatively, it has been proposed that "colour" is a general property of which "red" is an individual case. Moreover, a pure colour can be described by the corresponding wavelengths of light, with a band from 625 nm to 740 nm for red light. Hence, the qualitative property "colour" can be assigned a quantitative (or semi-quantitative) value such as 700 nm.
Qualitative value assignments can also differ according to the intended use. For example, "ethanol" may be treated as the identity of a specific compound or as an instance of a family of compounds with the general property "alcohols", which in turn can be seen as an instance of the general property "chemical species".
Seeing the progress and the increasing number of RMs characterized or certified for qualitative properties, ISO/REMCO decided in 2018 to start drafting internationally harmonized guidance for the production of such RMs.
This presents several issues with respect to terminology. The ISO Guide 30 definition of a certified reference material (CRM) requires it to have an RM certificate that provides the values of the specified properties, associated uncertainties and statements of metrological traceability. The relevance of metrological traceability to RMs with qualitative property values can, however, be unclear. Notes of the RM and CRM definitions in ISO Guide 30 clarify that the concept of value includes a qualitative property, such as identity or sequence, and that uncertainties for qualitative values can be expressed as probabilities or levels of confidence[18]. However, a qualitative property has no numerical value and at present ISO/IEC Guide 98-3 (GUM) provides no methodology for assigning an uncertainty to such property values. Metrological traceability is defined in ISO/IEC Guide 99 (VIM) as a property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations. Hence, it too is inapplicable to qualitative property values. Nevertheless, RMs with qualitative properties are needed now and are being produced in increasing numbers. In order to provide meaningful guidance on these aspects of RM production, this document:
— describes how confidence in qualitative assigned values can be expressed;
— describes the relevance of a documented unbroken chain of qualitative comparisons in assigning qualitative values by comparison with qualitative references or reference information;
— notes that determination of many qualitative properties depends on data obtained with quantitative measurements and that measurement uncertainty and metrological traceability are relevant to these measurements.
These issues are discussed in detail in 5.2.2 and 5.3.2.
General requirements, structural requirements, resource requirements and management requirements for the production of RMs are described in ISO 17034. These requirements apply to the production of RMs with qualitative properties. This document supplements ISO 17034 and related guidance on the production of RMs by providing additional guidance on the value assignment and the assessment of homogeneity, stability and commutability for RMs with qualitative properties.
