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GMP 13

Good Measurement Practice 

for

Ensuring Metrological Traceability1

1 Introduction

Purpose

The purpose of this Good Measurement Practice is to enable compliance with 

essential elements of Metrological Traceability. Traceability ensures that the 

measurements are accurate representations of the specific quantity subject to 

measurement, within the uncertainty of the measurement. 

To ensure metrological traceability, suitably calibrated standards that are 

appropriately maintained and cared for, proper standard operating procedures, 

continuous measurement control, surveillance, and suitable documentation must all 

be present. 

Test numbers issued by NIST should not be used nor required as proof of the 

adequacy or traceability of a test or measurement. Having a NIST number does not 

provide evidence that the measurement value provided by another organization has 

the property of metrological traceability. 

GMP 13 provides the basis for documenting metrological traceability. This GMP 

is a template that must be modified beyond Section 4 to match the laboratory scope, 

specific measurement parameters, and uncertainties in each laboratory.

Metrological Traceability is defined2 as the “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.”

NOTE 1: For this definition, a ‘reference’ can be a definition of a measurement

unit through its practical realization, or a measurement procedure including the 

measurement unit for a non-ordinal quantity, or a measurement standard.

NOTE 2: Metrological traceability requires an established calibration hierarchy.

NOTE 3: Specification of the reference must include the time at which this 

reference was used in establishing the calibration hierarchy, along with any other 

relevant metrological information about the reference, such as when the first 

calibration in the calibration hierarchy was performed.

 1 See also the International Laboratory Accreditation Cooperation (ILAC) Policy ILAC P10:01/2013, ILAC Policy 

on the Traceability of Measurement Results. https://ilac.org/publications-and-resources/ilac-policy-series/ (January 

2014). 2 These definitions are provided in the “International vocabulary of metrology — Basic and General Concepts and 

Associated Terms (VIM)” JCGM 100:2008 (2012).
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NOTE 4: For measurements with more than one input quantity in the measurement 

model, each of the input quantity values should itself be metrologically traceable 

and the calibration hierarchy involved may form a branched structure or a network. 

The effort involved in establishing metrological traceability for each input quantity 

value should be commensurate with its relative contribution to the measurement 

result.

NOTE 5: Metrological traceability of a measurement result does not ensure that 

the measurement uncertainty is adequate for a given purpose or that there is an 

absence of mistakes.

NOTE 6: A comparison between two measurement standards may be viewed as a 

calibration if the comparison is used to check and, if necessary, correct the quantity 

value and measurement uncertainty attributed to one of the measurement standards.

NOTE 7: The ILAC considers the elements for confirming metrological 

traceability to be an unbroken metrological traceability chain to an international 

measurement standard or a national measurement standard, a documented 

measurement uncertainty, a documented measurement procedure, accredited 

technical competence, metrological traceability to the SI, and calibration intervals 

(see ILAC P-10:2002).

NOTE 8: 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.

A Metrological Traceability Chain is defined as “traceability chain sequence of 

measurement standards and calibrations that is used to relate a measurement result 

to a reference”.

NOTE 1: A metrological traceability chain is defined through a calibration 

hierarchy. 

NOTE 2: A metrological traceability chain is used to establish metrological 

traceability of a measurement result. 

NOTE 3: A comparison between two measurement standards may be viewed as a 

calibration if the comparison is used to check and, if necessary, correct the quantity 

value and measurement unit.

Metrological Traceability to a Measurement Unit is defined as: metrological 

traceability to a unit metrological traceability where the reference is the definition 

of a measurement unit through its practical realization 

NOTE: The expression “traceability to the SI” means ‘metrological traceability to 

a measurement unit of the International System of Units’. 
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Prerequisites

Metrological traceability can be characterized by the following seven essential 

elements:

Realization of SI units. 

The measurand(s) must be defined. The primary national, international or 

intrinsic standards must be primary standards for the realization of the 

International System of Units (SI) (See GMP 13 and example standard 

hierarchies referencing the International System of Units (SI); See also 

NIST Special Publication 330, The International System of Units and NIST 

Special Publication 811, Guide for the Use of the International System of 

Units (SI));

Unbroken chain of comparisons. 

A documented system of comparisons with each step having the essential 

elements of metrological traceability going back to a standard acceptable to 

the parties, usually a national or international standard (See GMP 13 and 

example standard hierarchies referencing the International System of Units 

(SI);

Documented calibration program. 

Calibrations of standards (and equipment where appropriate) must be 

repeated at established (may be defined through measurement assurance) 

and appropriate intervals to preserve metrological traceability of the 

standard over time and use (see GLP 4, GMP 11);

Documented measurement uncertainty. 

The measurement uncertainty for each step in the traceability chain must be 

calculated according to defined methods and must be stated so that an 

overall uncertainty for the whole chain may be calculated (see SOP 29; See 

each calibration SOP published by the Office of Weights and Measures, 

which include detailed uncertainty budget tables);

Documented measurement procedure. 

Each step in the chain must be performed according to documented and 

validated procedures (see GMP 12) and the measurement results with 

expanded uncertainties must be documented (i.e., in a calibration certificate, 

see SOP 1); 

Accredited technical competence. 

The laboratories or bodies performing one or more steps in the chain must 

supply evidence of technical competence (e.g., by maintaining appropriate 
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training records, participating in interlaboratory comparisons, and by 

demonstrating that they are accredited by a recognized accreditation body; 

See GLP 1); and

Measurement assurance. 

A proper measurement assurance program must be established to ensure the 

validity of the measurement process and the accuracy of standard used at 

the time of the measurement (see SOPs 9, 17, 20, 30, and GLP 1).

Responsibility

Provider.

Providing support for the claim of traceability of the result of a 

measurement or value of a standard is the responsibility of the calibration 

provider. Calibration certificates must contain a statement regarding 

metrological traceability. See Appendices C and D for a form that may be 

used to assess evidence supporting metrological traceability.

User. 

Assessing the validity of a claim of traceability is the responsibility of the 

user of that result or value. Verifying claims of traceability often includes 

obtaining a calibration directly from a national metrology institute or 

another laboratory that has achieved recognition or accreditation through a 

recognized accrediting body. See Appendix E for a form that may be used 

to assess calibration certificates and standards for weights and measures 

applications. 

Use of, or reference to, official test numbers of a national metrology 

institute. 

Having an authentic test number does not provide assurance or evidence 

that the measurement value provided by another organization is traceable. 

Not only must there be an unbroken chain of comparisons, but each 

measurement must be accompanied by a statement of uncertainty associated 

with the value. Test report numbers should not be used nor required as proof 

of the adequacy or traceability of a test or measurement. National and 

international documentary standards for test and measurement quality 

requirements, such as ISO 10012, ISO/IEC 17025 and the ISO 9000 series,

provide guidance for assessing metrological traceability and do not require 

the use or reporting of specific test numbers to establish metrological 

traceability.

Safety

No outstanding safety concerns.
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2 Methodology

Summary

Traceability must be maintained through comparison to appropriate standards with 

suitable procedures and measurement uncertainties. Procedures are outlined in 

SOPs and GMPs. Examples of possible hierarchies of the standards leading to the 

metrological traceability of a calibration are provided in this document in Appendix 

A.

Procedure

Create Traceability Hierarchy Charts/Diagrams for the Laboratory.

The charts in Appendix A provide examples of possible traceability 

hierarchies for mass, length, volume, and temperature measurement 

disciplines. Each laboratory must define their exact traceability hierarchy in 

their quality management system (controlled laboratory documents) and 

have evidence of all essential elements of traceability confirmed to perform 

associated calibrations.

A worksheet is included as Appendix B to help in the definition and outline 

of the calibration system. The worksheet may be used as a template,

integrated with requirements for establishing calibration intervals from 

GMP 11, identify suitable calibration sources in the calibration program in 

a single controlled reference as part of the laboratory calibration program.

Periodically Assess Traceability Hierarchies and the Laboratory Calibration 

Program.

Example tools for assessing the laboratory’s objective evidence of 

metrological traceability are provided in Appendices C and D. Calibration 

laboratories need to have a calibration program that includes schedules for 

calibrations and regular reviews of documented evidence of metrological 

traceability. Regular assessments can provide ongoing confidence and 

assurance of metrological traceability. 

Appendix C – Traceability Evaluation Form – for assessing laboratory 

evidence of traceability;

Appendix D – Sample Technical Audit for Traceability Evidence – tool for 

selecting calibration certificates and assessing the evidence in the laboratory 

as part of a technical audit; and 

Appendix E – Evaluating Supplier Calibration Certificates for Metrological 

Traceability for Weights and Measures Applications – tool to assess 

traceability for weights and measures applications as needed.
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3 Calculations

There are no calculations in this GMP.

4 Assignment of Uncertainty

The uncertainty associated with reported calibration values is included within the 

uncertainty analysis for each SOP and in SOP 29, Calculating and Reporting Uncertainties.
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Appendix A – Examples

Mass - Example A

NIST

W 1 kg.

W 1 kg..

W 100 g.

W 100 g..

C 1 kg.

C 1 kg..

C 100 g.

C 100 g..

P 1 kg.

P 1 kg..

P 100 g.

P 100 g..

Client 

Echelon I, II

Calibration

Workload

Laboratory Field 

Level Working 

Standards

Client 

Echelon III

Calibration

Workload

Remaining 

Laboratory 

Standards

Metric/Avdp

Figure 1. Mass Example A (Echelon I).
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Mass - Example B

NIST

Monitoring kg., kg..

Primary Metric & 

Avdp Standards

Client 

Echelon I, II

Calibration

Workload

Laboratory Field 

Level Working 

Standards

Client 

Echelon III

Calibration

Workload

Laboratory 

Standards

Metric/Avdp

Figure 2. Mass Example B (Echelon II).
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Mass - Example C

NIST

Accredited

Laboratory

Laboratory Field 

Level Working 

Standards

Client 

Echelon III

Calibration

Workload

Laboratory 

Primary 

Standards

Metric/Avdp

Figure 3. Mass Example C (Echelon III).
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Volume - Example A

Volume Standards

5 gal, 25 gal, 

100 gal

NIST

Volume Group or 

Mass Group

Client Volume 

Transfer 

Calibration

Workload

(5 gal or larger)

Client

Gravimetric 

Volume Calibration

(5 gal or smaller)

Laboratory 

Mass Standards

Metric/Avdp

Mass Standards

Figure 4. Volume Example A (Echelon I).
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Volume - Example B

Volume Standards

5 gal, 25 gal, 

100 gal

NIST

Volume Group 

Client Volume 

Transfer 

Calibration

Workload

(5 gal or larger)

Volume Standards

Glassware

Standards

Client Volume 

Transfer 

Calibration

Workload

(Glassware)

Figure 5. Volume Example B (Echelon II).
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Length Example

18 in Rigid Rule

Primary Standard

NIST

Client 

Calibration

Workload

Length Bench

Working Standard

25 ft Tape

Primary Standard

100 ft Tape

Primary Standard

Client 

Calibration

Workload

Figure 6. Length Example.
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Temperature Example

NIST

SPRT

Primary Standard

Liquid-in-Glass

Primary Standard

Client 

Calibration

Workload

Laboratory Fixed 

Points

Laboratory

Working Standards

Figure 7. Temperature Example.
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Appendix B - Traceability worksheet

Parameter: Mass, Volume, Length, Temperature, Other

R SI (g, L, m, °C) Cal. date Source Interval Due Date

Range

Customary (lb, gal, ft, °F)

Range

W SI (g, L, m, °C) Cal. date Source Interval Due Date

Range

Customary (lb, gal, ft, °F)

Range

Sc

SI (g, L, m, °C) Cal. date Source Interval Due Date

Range

Customary (lb, gal, ft, °F)

Range

R = Reference Standards; W = Working Standards; Sc = Check Standards.
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Appendix C: Traceability Evaluation Form

Assess whether all of the essential elements of traceability are available for the entire Scope by 

completing the table below. Specific Analysis: Include specific comments on what was observed and 

what evidence is available.

Manual 

Reference Description Specific Example or Evidence Complies?Yes/No

Realization to SI Units.

Unbroken Chain of 

Evidence.

Standards (Reference, 

Working, and Check or 

Control) demonstrate an 

unbroken chain to SI

references.

Calibration program 

with suitable intervals.

No calibrations are “past 

due.”

Documented 

Uncertainty. Needed for 

each step in the 

hierarchy.

Documented 

Procedures. 

Suitable, applicable, 

documented and 

validated procedures.

Technical Competency. 

Demonstration of 

competency is 

documented.

Measurement Assurance.
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Appendix D: Sample Technical Audit for Traceability Evidence

Select 3 calibration certificates for the past year, and identify the following specific information to 

support traceability for each calibration. On each chart, next to each block for your primary and working 

standards, please list evidence that is available in your laboratory:

Item Evaluated Evidence

Calibration selected (Measurement 

Type, Lab Test Number)

Test Number(s) for all standards used

Calibration Date(s) for the standards

Calibration providers have been 

evaluated and monitored and are 

accredited

Calibration program and intervals are 

stated and up to date

Uncertainty analysis components 

included(s)

Uncertainty file name where data is 

stored

Documented SOP used

Evidence of competency

Measurement assurance type 

Measurement Assurance file name 

where data is stored

Certificates contain a Traceability 

Statement

You should be able to identify any gaps and corrective action needed in your laboratory in the area of 

traceability, uncertainty analysis, and measurement assurance. Note actions on laboratory action forms.
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Appendix E: Evaluating Supplier Calibration Certificates for Metrological Traceability 

for Weights and Measures Applications

Step Notes Accept 

(Yes/No)

Legal

Does your law allow acceptance of reports for the applicable use 

(laboratory standards, field standards, service company standards)?

Does your law specify traceability to NIST? To the SI? 

Is there a required calibration interval?

Specifications

Does your law require assessment against published specifications 

like the NIST Handbook 105-series documentary standards? IF 

SO, have the customer standards been evaluated for compliance to 

specifications (are the standards suitable for use?) Note: This is 

NOT common for non-governmental laboratories, even when 

accredited, and cannot be done if there are no applicable 

specifications. Accreditation has nothing to do with evaluating 

compliance to specifications.

Documented 

Calibration 

Certificate

Is there a valid calibration certificate for all of the standards? Does 

it (do they) comply with laboratory requirements? (Checklist 

available.)

Specifically, traceability components: reference to SI and standards 

used, traceability statement, uncertainty statement, measurement 

results, measurement uncertainty, and reference to documented 

procedures.

Is the report from a 

National Metrology 

Laboratory (NMI) 

like NIST?

Does your law permit you to accept a calibration certificate for a 

standard that was tested by Measurement Canada that is traceable 

to NRC for example?

Has that NMI signed the CIPM MRA and is this range, scope, 

uncertainty of measurements published in the Appendix C listing 

maintained by BIPM? (List available.)

Is the report from a 

NIST OWM 

Recognized 

laboratory?

OWM is NOT a recognized Accreditation Body and has not signed 

the ILAC MRA.

Is the measurement parameter on the laboratory Scope?

Is the reported uncertainty acceptable for your needs?

(Posted on the NIST website, includes those Accredited by 

NVLAP.)

Is the report from 

an Accredited 

laboratory?

Is the Accreditation Body a signatory of the ILAC MRA?

(List available.)

Is the measurement parameter on their Scope now?

Is the currently reported uncertainty small enough to meet 

requirements for the laboratory or for Fundamental Considerations 

(less than 1/3 the applicable tolerance)?? (Directories available

with Accreditation Bodies.)