I Frequently Asked Questions

I.A Questions about Metrological Traceability in General

I.A.1 What is traceability?

The definition of traceability that has achieved global acceptance in the metrology community is contained in the International vocabulary of metrology - Basic and general concepts and associated terms [1]: "…property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty

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 specified references. The measurement system by which values and uncertainties are transferred must be clearly understood and under control.

I.A.2 Is it correct to say that measurements or standards are traceable?

Only measurement results are traceable.

I.A.3 Is it correct to say that an organization is traceable?

Organizations cannot be traceable. Only measurement results can be traceable.

I.A.4 Who is responsible for supporting claims of traceability?

The provider of the result of a measurement 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 organization.

I.A.5 What do I need to do to support a claim of traceability?

To support a claim, the provider of a measurement result must document the measurement process or system used to establish the claim and provide a description of the chain of calibrations that were used to establish a connection to a particular specified reference. There are several common elements to all valid statements or claims of traceability:

• a clearly defined particular quantity that has been measured
  
  
• a complete description of the measurement system or working standard used to perform the measurement
  
  
• a stated measurement result, which includes a documented uncertainty
  
  
• a complete specification of the reference at the time the measurement system or working standard was compared to it
  
  
• 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
  
  
• an ‘internal measurement assurance’ program for establishing the status of the specified 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.

For information and guidance on expressing measurement uncertainty, see http://physics.nist.gov/cuu/.

See Section V Checklist for Traceability through Calibration for additional guidance.

I.A.6 Who is responsible for assessing the validity of claims of traceability?

The user of the result of a measurement is responsible for assessing the validity of a claim of traceability. However, as discussed in Question I.A.4above, the provider is responsible for providing the necessary information that the user assesses.

I.A.7 What should I look for in a valid claim of traceability?

See Question I.A.5 above, which provides information on what the provider of a measurement result should do to support a claim of traceability. As a user, you should look for these elements. See Section V Checklist for Traceability through Calibration for additional guidance.

I.B Questions about Establishing Metrological Traceability

I.B.1 What is involved in establishing metrological traceability?

Per the VIM definition, metrological traceability is established through "an unbroken chain of calibrations, each contributing to the measurement uncertainty." In practical terms, this means having in place, for each link in the chain:

• a clearly defined particular quantity that has been measured
  
  
• a complete description of the measurement system or working standard used to perform the measurement
  
  
• a stated measurement result or value, with a documented uncertainty
  
  
• a complete specification of the specified reference at the time the measurement system or working standard was calibrated against it
  
  
• 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
  
  
• an ‘internal measurement assurance’ program for establishing the status of the specified reference at the time that the measurement system or working standard was calibrated against 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.

For information and guidance on expressing measurement uncertainty, see http://physics.nist.gov/cuu/.

I.B.2 Who is responsible for establishing the traceability of measurement results?

The provider of the result of a measurement is responsible for establishing the traceability of this result. This is the case whether that provider is NIST or another organization.

I.B.3 Does metrological traceability need to be reexamined periodically? What are the criteria for judging the integrity of the metrological traceability chain?

Yes, traceability does need to be reexamined periodically, and the criteria for assessing the integrity of the traceability chain are dependent on many things. To be considered are: the measurement requirements, the needs of the client, the dependability of the equipment and standards, the environmental effects, etc. For more details, see Reference [2].

I.B.4 Is a NIST Test Report Number necessary and/or sufficient evidence of traceability?

Test report numbers issued by NIST are intended to be used solely for administrative purposes. Although they are often used to uniquely identify documents which bear evidence of traceability, test report numbers themselves do not address the issues listed in I.B.1 above, and should not be used nor required as the sole proof of traceability.

I.B.5 To establish the traceability of a measurement result for a particular measurand, is it also necessary to establish the traceability of all other values of measured quantities associated with the measurand, especially those that might contribute in a relatively minor way? For example, when making a dimensional measurement, do the measurements of barometric pressure, humidity, and temperature for a dimensional measurement need to be traceable to the International System of Units (SI)?

As a general principle, the definition of metrological traceability in the VIM suggests that it is necessary to establish the traceability of all associated input quantities in a measurement model. A measurement model defines the way in which a measurement result depends upon an associated input quantity. For example, a dimensional measurement result may depend upon barometric pressure, humidity, and temperature, if the value of, and/or the variation in these quantities contributes significantly to the value or stated uncertainty of the measurement result.

As a practical matter, the contribution of an input quantity to a measurement result is significant if a change in the value or uncertainty of the input quantity corresponds to a change in the significant figures of the stated value or uncertainty of the measurement result. When this is the case, traceability for the measurement of the associated input quantity should be established. The required levels of uncertainty that must be achieved for these associated input quantities may be relatively large, depending on the degree to which the input quantity actually affects the value or the stated uncertainty of the measurement result.