Asas

LAB 12 5 The expression of uncertainty in testing

The Expression of Uncertainty in Testing
[UKAS Publication ref: LAB 12]

Contents
Section 1 2 3 4 5 6 7 8 9 10 11 Introduction Definitions Reporting and evaluation of uncertainty Reasons for evaluating uncertainty General principles Sources of uncertainty Estimation of uncertainty Summary of the steps in estimating uncertainty Method ofstating results Assessment of compliance with specification Proficiency testing and measurement audit Definitions of terms Page 3 3 3 4 4 5 6 8 9 10 12 13

Appendix A

Edition 1 5 October 2000

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LAB 12 Z The expression of uncertainty in testing

About the United Kingdom Accreditation Service
The United Kingdom Accreditation Service (UKAS) is recognised by the UK Government asthe national body responsible for assessing and accrediting the competence of organisations in the fields of calibration, testing, inspection and certification of systems, products and personnel.

LAB 12 Z Edition 1 Z October 2000 United Kingdom Accreditation Service 21 – 47 High Street Feltham Middlesex TW13 4UN UK Tel: 020 8917 8555 Fax: 020 8917 8500 website: www.ukas.com © United KingdomAccreditation Service 2000

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Edition 1 5 October 2000

LAB 12 5 The expression of uncertainty in testing

1
1.1

Introduction
The general requirements for the estimation and reporting of uncertainty in accredited laboratories are given in ISO/IEC 17025. General guidance on how they may be met when estimating and reporting uncertainty in testing is given in this publication.UKAS Publications, M4 lists some other publications giving guidance on evaluation of uncertainty. The general approach to evaluating and expressing uncertainty in testing outlined here is based on the recommendations produced by the International Committee for Weights and Measures (CIPM), as described in the Guide to the Expression of Uncertainty in Measurement, 1993, ISO Geneva. (The Guide). Inthe interests of keeping to a general, descriptive format, exceptions, special cases and qualifying remarks have not been dealt with in detail and equations have not been included. The Guide should be consulted for equations and symbols, and when needing to resolve special difficulties that may arise in specific tests.

1.2

1.3

2
2.1

Definitions
Definitions of terms are given inalphabetical order in Appendix A. Defined terms are printed in bold at the first appropriate occurrence in the main body of the text.

3
3.1 3.2

Reporting and evaluation of uncertainty
The requirements of ISO/IEC 17025 distinguish between the need for reporting and the need for evaluation of uncertainty of measurement. Reporting is required when information on uncertainty is relevant to thevalidity or application of the test results, when the client requires it or when the uncertainty affects compliance with a specification limit. Evaluation of uncertainty of measurement is required for calibrations, including those performed in house, and procedures for estimating uncertainty of measurement of tests are needed and need to be applied. The complexity of tests may in some cases preclude arigorous evaluation of uncertainty. In such cases, at least a list of the potential contributors to uncertainty should be made and should include reasonable estimates of the magnitude of each component uncertainty. These estimates may be based on previous experience and make use of data from method validation and other sources, such as quality control data, proficiency test data and round robintest results. It is recommended that an example be produced in which the overall uncertainty of the result for that test method is calculated. In cases where a well-recognised test method specifies limits to the values of major sources of uncertainty of measurement and specifies the form of presentation of the results the requirement to estimate uncertainty of measurement can be considered to...