Measurement Traceability
What is traceability?
The definition of traceability that has achieved global acceptance in the metrology community is contained in the International Vocabulary of Basic and General Terms in Metrology (VIM; 1993):
" the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties."
It is important to note that traceability is the property of the result of a measurement, not of an instrument or calibration report or laboratory. It is not achieved by following any one particular procedure or using special equipment. Merely having an instrument calibrated, even by NIST, is not enough to make the measurement result obtained from that instrument traceable to realizations of the appropriate SI unit or other stated references. The measurement system by which values are transferred must be clearly understood and under control.
What do I need to do to support a claim of traceability?
Only measurement results and values of standards can be traceable. Organizations cannot be traceable. The provider of the result of a measurement or value of a standard is responsible for supporting its claim of the traceability of that result or value. This is the case whether that provider is NIST or another National Metrological Institute (NMI).
To support a claim of traceability, the provider of a measurement result or value of a standard must document the measurement process or system used to establish the claim and provide a description of the chain of comparisons that were used to establish a connection to a particular stated reference. There are several common elements to all valid statements or claims of traceability:
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A clearly defined particular quantity that has been measured
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A complete description of the measurement system or working standard used to perform the measurement
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A stated measurement result or value, with a documented uncertainty
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A complete specification of the stated reference at the time the measurement system or working standard was compared to it
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An internal measurement assurance program for establishing the status of the measurement system or working standard at all times pertinent to the claim of traceability
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An internal measurement assurance program for establishing the status of the stated reference at the time that the measurement system or working standard was compared to it
An internal measurement assurance program may be quite simple or very complex, the level or rigor to be determined depending on the level of uncertainty at issue and what is needed to demonstrate its credibility. Users of a measurement result are responsible for determining what is adequate to meet their needs.
Is a NIST Test Report Number sufficient evidence of traceability?
Test report numbers issued by NIST are used solely for administrative purposes. Although they often uniquely identify documents that bear evidence of traceability, test report numbers themselves do not address the requirements, and should not be considered as the sole evidence of traceability.
What is meant by the phrase "traceable to NIST"?
According to the internationally recognized VIM definition, traceability is a property of the result of a measurement or the value of a standard by which that result or value is related to standards, not to institutions. Accordingly, the phrase "traceable to NIST", in its most proper sense, is shorthand for "results of measurements that are traceable to reference standards developed and maintained by NIST".