Traceable measurement depends on valid uncertainty evaluation, and all uncertainty evaluation depends on a measurement model and a computational approach to perform the evaluation. The 'Guide to the expression of uncertainty in measurement' (GUM) is the primary document regarding measurement uncertainty evaluation. It provides a framework and procedure for evaluating and expressing measurement uncertainty that is applied at National Metrology Institutes, calibration and testing laboratories, and in industry.

Working Group 1 'Expression of Uncertainty in Measurement (GUM)' of the Joint Committee for Guides in Metrology (JCGM) has responsibility for maintaining and promoting the GUM. It has published two specific guidance documents (or Supplements to the GUM) on, respectively, a Monte Carlo method for measurement uncertainty evaluation and extensions to any number of measurands that extend the applicability of the GUM to a wider set of uncertainty evaluation problems. It is also planned to produce a further Supplement on developing and using measurement models in the context of uncertainty evaluation. The Supplements are part of a set of documents that include published documents giving an introduction to the GUM and on the role of measurement uncertainty in conformity assessment, as well as planned documents describing concepts and basic principles and on applications of the least-squares method. The documents are published by the member organizations of the JCGM, which are BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP and OIML.

The project will make major contributions to JCGM-WG1 to complete: (a) a new edition of the GUM (JCGM 100); (b) an examples document (JCGM 110); and (c) a modelling supplement (JCGM 103), and to influence other JCGM documents. Knowledge transfer activities, in the form of papers, articles, presentations, training and software, will be undertaken to support users in applying the JCGM documents. The project will seek to influence and support other international and national standardisation and accreditation bodies, including ISO, BSI and UKAS, especially through leadership of relevant ISO and BSI committees, and relationships with UKAS.

Papers and reports:

• W. Bich, M.G. Cox and P.M. Harris. The evolution of the 'Guide to the expression of uncertainty in measurement' (Metrologia 43:2006)
• M.G. Cox and P.M. Harris. Measurement uncertainty and traceability (Meas. Sci. Techn. 17:2006)
• M.G. Cox, P.M. Harris, G. Nam and D. Thomas. The use of a Monte Carlo method for uncertainty calculation, with an application to the measurement of neutron dose equivalent rate (Radiation Protection Dosimetry: 2006)
• T.J. Esward, A. de Ginestous, P.M. Harris, I.D. Hill, I.M. Smith, B.A. Wichmann and E.R. Woolliams. A Monte Carlo method for uncertainty evaluation implemented on a distributed computing system (Metrologia 44:2007)
• A.B. Forbes, M.G. Cox, G.B. Rossi and P.M. Harris. A probabilistic approach to the analysis of measurement processes (Metrologia 45:2008)
• G. Wübbeler, P.M. Harris, M.G. Cox and C. Elster. A two-stage procedure for determining the number of trials in the application of a Monte Carlo method for uncertainty evaluation (Metrologia 47:2010)
• M.G. Cox, P.M. Harris and I.M. Smith. Software specifications for uncertainty evaluation (NPL Report MS 7: 2010)
• M.G. Cox and P.M. Harris. Software Support for Metrology Best Practice Guide No 6: Uncertainty evaluation (NPL Report MS 6: 2010)
• C. Matthews, P. Clarkson, P. M. Harris, W. G. Kürten Ihlenfeld and P. S. Wright. Evaluation of flicker measurement uncertainties by a Monte Carlo method (IEEE Transactions on Instrumentation and Measurement 60:2011)
• W. Bich, M.G. Cox, R. Dybkaer, C. Elster, W.T. Estler, B. Hibbert, H. Imai, W. Kool, C. Michotte, L. Nielsen, L. Pendrill, S. Sidney, A.M.H. van der Veen and W. Wöger. Revision of the 'Guide to the Expression of Uncertainty in Measurement' (Metrologia 49:2012)
• S. Eichstädt, A. Link, P.M. Harris and C. Elster. Efficient implementation of a Monte Carlo method for uncertainty evaluation in dynamic measurements (Metrologia 49:2012)
• P.M. Harris, C.E. Matthews, M.G. Cox and A.B. Forbes. Summarizing the output of a Monte Carlo method for uncertainty evaluation (Metrologia 51:2014)