NIST Policy on Metrological Traceability

NIST has an organizational policy on traceability and a set of related supplementary materials, which includes answers to questions frequently asked by customers of NIST measurement services. The policy and supplementary materials are intended to serve as a resource for NIST customers.

Introduction

The mission of NIST is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life. To help meet the measurement and standards needs of U.S. industry and the nation, NIST provides calibrations, standard reference materials, standard reference data, test methods, proficiency evaluation materials¹ [1], tools that facilitate the evaluation of measurement uncertainty [2], measurement quality assurance programs, and laboratory accreditation services that assist a customer in establishing traceability of measurement results.

Metrological traceability [3] requires the establishment of an unbroken chain of calibrations to specified reference standards: typically national or international standards, in particular realizations of the measurement units of the International System of Units (SI). NIST assures the traceability to the SI, or to other specified standards, of measurement results that NIST itself provides, either directly or through an official NIST program or collaboration. Other organizations are responsible for establishing the traceability of their own results to those of NIST or other specified references. NIST has adopted this policy statement to document the NIST role with respect to traceability.

Policy Statement

To support the conduct of its mission and to ensure that the use of its name, products, and services is consistent with its authority and responsibility, NIST:

1. Adopts for its own use and recommends for use by others the definition of metrological traceability provided in the most recent version of the International vocabulary of metrology – Basic and general concepts and associated terms (VIM[4], 2.41): "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."
2. Establishes metrological traceability to the SI, or to other specified standards, of its own measurement results provided to customers in certificates of NIST calibrations, reference materials, and other measurement services, that operate in accordance with the NIST Quality System for Measurement Services.
3. Asserts that providing support for a claim of metrological traceability of a measurement result is the responsibility of the provider of that result; whether that provider is NIST or another organization; and that assessing the validity of such a claim is the responsibility of the user of that result.
4. Communicates, especially where claims expressing or implying the contrary are made, that NIST does not define, specify, assure, or certify metrological traceability of the results of measurements except those that NIST itself provides, either directly or through an official NIST program or collaboration.
5. Emphasizes that traceability alone does not signify or guarantee fitness for purpose, because this typically requires that the uncertainty associated with a measured value or calibration be sufficiently small to satisfy a particular measurement need.
6. Collaborates on development of standard definitions, interpretations, and recommended practices with organizations that have authority and responsibility for variously defining, specifying, assuring, or certifying metrological traceability.
7. Develops and disseminates technical information on traceability, provides tools and expertise supporting the evaluation of measurement uncertainty, and conducts coordinated outreach programs on issues of traceability and related requirements.
8. Assigns responsibility for oversight of implementation of the NIST policy on metrological traceability to the NIST Measurement Services Council (NMSC).

In text references: 

[1] Homogeneous materials or artifacts that are used to test and evaluate the measurement performance and fitness for purpose of measuring systems (VIM §3.2) or measurement procedures (VIM §2.6).

[2] NIST Uncertainty Machine and NIST Consensus Builder, with user’s manuals available online, and supported by peer-reviewed publications: Lafarge & Possolo (NCSLI Measure Journal of Measurement Science 10(3): 20-27); Koepke et al. (2017, Metrologia 54(3): S34-S62, DOI 10.1088/1681-7575/aa6c0e).

[3] The abbreviated term “traceability” is sometimes used to mean “metrological traceability” as well as other concepts, such as “sample traceability” or “document traceability” or “instrument traceability” or “material traceability”, where the history (“trace”) of an item is meant. Therefore, the full term of “metrological traceability” is preferred if there is any risk of confusion (VIM §2.41, Note 8).

[4] Joint Committee for Guides in Metrology (2012, 3rd Edition, International Bureau of Weights and Measures (BIPM)). The VIM is freely available online at https://jcgm.bipm.org/vim/en/.

Additional references:

• Joint BIPM, OIML, ILAC, and ISO Declaration on Metrological Traceability
• CIPM 2009-24, Traceability in the CIPM MRA

¹Terms are defined in Metrological Traceability: Frequently Asked Questions and NIST Policy