LGC AXIO Proficiency Testing - 2023 Water & Environmental Catalogue
About reporting and evaluating results
What is the estimated uncertainty of the assigned value? The assigned value has a standard uncertainty (ux) that depends upon the method used to derive the assigned value. When the assigned value is determined by the consensus of participants’ results, the estimated standard uncertainty of the assigned value can be calculated by: ux = 1.25 x Robust standard deviation/ √ n Where n = number of results When the assigned value is determined by formulation, the standard uncertainty is estimated by the combination of uncertainties of all sources of error, such as gravimetric and volumetric measurements. If ux is ≤ 0.3 x SDPA, then the uncertainty of the assigned value can be considered negligible and need not be considered in the interpretation of results. If ux is > 0.3 x SDPA, then the uncertainty of the assigned value is not negligible in relation to the SDPA and so z’ (z-prime) performance scores, which take into account the standard uncertainty of the assigned value in their calculation, will be reported in place of z scores. How is a z’ (z-prime) score calculated? A z’ score incorporates the standard uncertainty of the assigned value and is calculated as follows: z’ = (x - X) √ ( SDPA 2 + U x 2 ) Where x = participant result X = the assigned value SDPA = Standard Deviation for Proficiency Assessment ux = standard uncertainty of the assigned value X A z’ score is interpreted in exactly the same way as a z score, ≤2 is satisfactory, >2 but <3 is questionable and ≥3 is unsatisfactory.
Do you include outlying results due to ‘errors and blunders’ in the statistical analysis of the data? Although robust estimators are used in order to minimise the influence of outlying results, extreme results or results that are identifiably invalid should not be included in the statistical analysis of the data. For example, these may be results caused by calculation errors or the use of incorrect units. However, such results can be difficult to identify by the PT provider. For this reason, the robust mean and standard deviation will be calculated in the usual way, but those results that are out of the range of the assigned value ± 5 x SDPA will be excluded and the robust mean and standard deviation will then be recalculated. These recalculated values will be used for the statistical analysis. By removing these ‘blunders’ from the dataset any influence on the summary statistics is minimised. All results, including excluded results, will be given performance scores. How can I graphically plot and analyse trends for qualitative results? Qualitative results are difficult to depict graphically as they are not normally allocated a performance score. However for qualitative results, a correct result could be allocated a performance score of 0 to represent a satisfactory result. A false positive result can be represented by a performance score of + 3, whilst a false negative result can be represented by a performance score of - 3. If plotted graphically over time, this should give a clear visual indicator of Following statistical evaluation of the results, the reports will generally be available on the website within 4 to 10 working days of round closure (see specific scheme description). We aim to provide 95% of our reports to participants within 5 working days. Participants will be emailed when the report is available. The content of reports vary from scheme to scheme but include details of the composition of test materials, the assigned values, and tabular and / or graphical representations of participants’ results. performance in qualitative tests. How will I receive my report?
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