The aims of this project are to ensure that the uncertainties associated with measured values are credible and reliable and are based on objective and defensible methods.

Aims and Rationale

A measured value is meaningless without a quantitative statement of its quality in the form of an uncertainty. The need to evaluate and express the uncertainty associated with a measured value applies generally across all areas of measurement, and arises in the applications of measurement to standardisation, calibration and testing, laboratory accreditation and in support of trade and regulation. The project will maintain the UK NMS at the forefront of the world metrology community in the area of uncertainty evaluation, and ensure UK industry and national organizations such as BSI and UKAS, are given early and privileged access to the results of international activity in the area.

The Guide to the Expression of Uncertainty in Measurement (GUM) is the primary document regarding uncertainty in measurement, and provides a framework for uncertainty evaluation that is applied across the NMS and throughout industry. Although formally only a guide, the GUM has the status of a de facto standard, and as such it underpins many national and international documents and standards with which organizations involved in measurement are required to comply. In the UK, UKAS maintain a guidance document, M3003, which assists their accredited laboratories in preparing uncertainty budgets in accordance with the GUM.

The GUM has recognised limitations and deficiencies. Working Group 1, Measurement Uncertainty, of the Joint Committee of Guides in Metrology (JCGM/WG1) has the task of producing revisions of the GUM and supporting documents, in order to address these limitations and to put the GUM on a more secure probabilistic basis. As part of previous SSfM programmes, the UK NMS has led the drafting of the first supporting document to the GUM (GUM Supplement 1) to be produced by JCGM/WG1.

Traceability at the international level is supported by interlaboratory comparisons (ILCs), and Key Comparisons (KCs), which are used to establish the equivalence of the measurements made by different NMIs. The Mutual Recognition Arrangement (MRA) relies on the results of KCs and the evaluation of interlaboratory comparison data. Additionally there are new challenges of linking these comparisons with others coordinated by Regional Metrology Organisations and the treatment of artefact drift during a comparison.  The statistical analysis to investigate the equivalence of the results from the different participants in an ILC requires a harmonised and practical approach.

Objectives

The project will:

• Support and extend the application of the GUM in UK industry and the NMS to cover, for example, measurements (a) at the limit of detection, (b) described by non-linear models, and (c) with a general number of measurands.
• Collaborate with UK industry and NMS metrologists to promote generic and scientifically based methods for uncertainty evaluation through participation in case studies.
• Develop methods, with supporting algorithms and software, for uncertainty evaluation for measurement systems and models that have a general number of measurands, such as arise in electrical and acoustical measurement where the output quantities may be complex-valued and in dimensional measurement where the output quantities may describe the position of an artefact.
• Develop a model-based framework for ILCs that is consistent with the GUM, undertaking case studies including simulation studies and participation in actual ILCs.
• Develop algorithms and software for the analysis of ILC data, accounting for the design of the experiment and the linking of results to additional comparison loops.
• Ensure that the UK NMS continues to lead national and international work in the area.