LGC AXIO Proficiency Testing - 2023 Water & Environmental Catalogue
About reporting and evaluating results
Can I use PT data to estimate my measurement uncertainty? It is possible, but must be regarded as a very rough estimate, and is not an approach addressed in many guides to evaluating measurement uncertainty. However documents that do address the use of PT data are: • EURACHEM/CITAC Guide ‘Quantifying Uncertainty in Analytical Measurement’ (available at www. eurachem.org) • NORDTEST Report TR 537 ‘Handbook for Calculation of Measurement Uncertainty in Environmental Laboratories’ • ISO 19036 ‘Microbiology of Food and Animal Feeding Stuffs - Guidelines for the Estimation of Measurement Uncertainty for Quantitative Determinations’. What is the Standard Deviation for Proficiency Assessment (SDPA)? The SDPA expresses the acceptable difference between the laboratory result and the assigned value. An acceptable z performance score represents a result that does not deviate from the assigned value by more than twice the SDPA. The method used to determine the SDPA may vary depending upon the particular PT scheme and test material and is detailed in the relevant scheme description. A fit for purpose value for SDPA, rather than being derived from participant results, is preferable as this enables z scores to be compared from round to round to demonstrate general trends. For each scheme, the value of SDPA and the method used to derive it is reported in the scheme description and/or report.
How is the assigned value established? ISO 13528: ‘Statistical Methods for use in Proficiency Testing by Interlaboratory Comparisons’ sets out how the assigned value and performance assessment criteria can be established and describes the options for the various performance scoring systems. The assigned value is the value selected as being the best estimate of the ‘true value’ for the parameter under test. The method used to determine the assigned value may vary depending upon the particular PT scheme and test material and is detailed in the relevant scheme description. For quantitative tests, where it is appropriate, practicable and technically feasible, the assigned value will be derived through formulation (or occasionally through the use of a certified reference material) to provide metrological traceability; the associated uncertainty of the value can therefore be estimated. However, in many cases the use of a consensus value is the only practicable and technically feasible approach to use. When the assigned value is determined from the consensus value of participant results, or from expert laboratories, robust statistical methods are used for calculation of the consensus value, the estimated standard uncertainty and the robust standard deviation. For qualitative tests, participant results are compared against the intended result (assigned value) based on formulation or expert assessment. For interpretive schemes where the result is subjective rather than quantifiable, a model answer produced by appropriate experts will be published in the report. For microbiology test materials, all participant results are transformed by converting them to log10 before the statistical analysis is undertaken. How do I evaluate measurement uncertainty? The aim when evaluating measurement uncertainty is to combine the effects of all the errors that will influence the measurement result, into a single value. There are many different guides available which provide advice on evaluating measurement uncertainty. There are two specific guides that are internationally recognised: • ISO (BIPM, IEC, IFCC, IUPAC, IUPAP and OIML) ‘Guide to the Expression of Uncertainty in Measurement’ • EURACHEM/CITAC Guide ‘Quantifying Uncertainty in Analytical Measurement’ (available at: www. eurachem.org). Further information on approaches to evaluating measurement uncertainty may also be available from your national accreditation body.
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