An approach for more precise statements of metrological traceability on reference material certificates

Metrological traceability of measurement results is a key requirement for the comparability of such data in time and space and with legal or product specifications. Similarly, traceability of the certified values of a certified reference material (CRM) is a prerequisite for their usefulness. Consequently, a statement on metrological traceability of the certified value of the specified property is a requirement of the ISO Guide series on Reference Materials. The establishment of the traceability of a value assigned to a property of a reference material to a stated reference is pivotal to the use of the material. Metrological traceability is related to a combination of four attributes that are assigned to a clearly identified sample: description of the property, description of the quantity/kind of quantity, a number with its corresponding uncertainty and the measurement unit. The first two items define the identity of the measurand, whereas the latter two describe the value that is assigned to this measurand. Therefore, the concept of traceability rests on two anchor points, namely identity and quantity value. The identity of a measurand can be defined by its structure alone or by an operational/procedural definition, the quantity value of the measurand can refer to the SI, or to an artefact, if standards of arbitrary values have been used for the calibration of equipment (e.g. WHO standards; international units). The presented approach splits traceability statements for CRMs into a description of the identity of the measurand and the quantity value and provides a solution to overcome the conflict as to whether a measurement result should be declared as traceable to the SI or traceable to a method.