5 Elements of traceability 5.1 Traceability is characterised by a number of essential elements: (a) an unbroken chain of comparisons going back to a standard acceptable to the parties, usually a national or international standard; (b) measurement uncertainty; the measurement uncertainty for each step in the traceability chain must be calculated according to agreed methods and must be stated so that an overall uncertainty for the whole chain may be calculated; (c) documentation; each step in the chain must be performed according to documented and generally acknowledged procedures; the results must equally be documented; (d) competence; the laboratories or bodies performing one or more steps in the chain must supply evidence for their technical competence, eg by demonstrating that they are accredited; (e) reference to SI units; the chain of comparisons must end at primary standards for the realization of the SI units; (f) re-calibrations; calibrations must be repeated at appropriate intervals; the length of these intervals will depend on a number of variables, eg uncertainty required, frequency of use, way of use, stability of the equipment. แหล่งที่มาของข้อมูล EAL – G12:1995 Traceability of Measuring and Test Equipment to National Standards ( Page 6 )
