ISO 17511:2003 specifies how to assure the metrological traceability of values assigned to calibrators and control materials intended to establish or verify trueness of measurement. The calibrators and control materials are those provided by the manufacturers as part of, or to be used together with, in vitro diagnostic medical devices. External quality assessment (survey) samples, with proven commutability, whose values have been assigned by means of internationally agreed reference measurement systems or internationally agreed conventional reference measurement systems fall within the scope of ISO 17511:2003. ISO 17511:2003 is not applicable to control materials that do not have an assigned value and are used only for assessing the precision of a measurement procedure, either its repeatability or reproducibility (precision control materials); control materials intended for intralaboratory quality control purposes and supplied with intervals of suggested acceptable values, each interval obtained by interlaboratory consensus with respect to one specified measurement procedure, and with limiting values that are not metrologically traceable; correlation between results of two measurement procedures at the same metrological level, purporting to measure the same quantity, because such "horizontal" correlation does not provide metrological traceability; calibration derived from correlation between the results of two measurement procedures at different metrological levels, but with quantities having analytes of different characteristics; metrological traceability of routine results to the product calibrator and their relations to any medical discrimination limit; and properties involving nominal scales, i.e. where no magnitude is involved (e.g. identification of blood cells).