A CRM is a reference material, characterised by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that provides the value of the specified property, its associated uncertianty, and a statement of metrological traceability.
The difference between a CRM and an AMIS reference material is solely in the last two requirements (uncertainty and traceability). From May 2009 AMIS certificates have fulfilled the CRM requirements by including the Combined Standard Uncertainty and a statement on the Metrological Traceability.
Combined standard uncertainty: The samples used in this certification process have been selected in such a way as to represent the entire batch of material and were taken from the final packaged units; therefore all possible sources of uncertainty (sample uncertainty and measurement uncertainty) are included in the final combined standard uncertainty determination. The uncertainty measurement takes into consideration the between lab and the within lab variances and is calculated from the square roots of the variances of these components using the formula:
Metrological Traceability: The values quoted in our certificates are based on the consensus values derived from statistical analysis of the data from an inter laboratory measurement program. Traceability to SI units is via the standards used by the individual laboratories the majority of which are accredited and who have maintained measurement traceability during the analytical process.