Proficiency testing catalogue - LGC Standards

Q: How do I interpret my results?A: For quantitative examinations, participantperformance is assessed using the z-score,and the following interpretation is given toresults.|z| ≤ 2.00Satisfactory result2.00 < |z| < 3.00 Questionable result|z| ≥ 3.00Unsatisfactory resultFor qualitative examinations, laboratoriesreporting the assigned result will beconsidered correct.Q: What are the advantages of a z-score?A: • Results can be expressed in a form that isrelatively easy to interpret and understand;• Results can be summarised in graphical ortabular form to depict overall performance;• A performance score allows participants todirectly compare their own result with others;• If consistent statistical values are applied,a performance score enables participantsto monitor and trend their own performanceover time.It is important to interpret any performancescore in the full context of the overall resultsand in the context of a laboratory’s ownquality control measures.of all sources of error, such as gravimetricand volumetric measurements.If u x is ≤ 0.3 x SDPA, then the uncertaintyof the assigned value can be considerednegligible and need not be considered in theinterpretation of results.If u x is > 0.3 x SDPA, then the uncertainty ofthe assigned value is not negligible in relationto the SDPA and so z’- scores (z-prime),which takes into account the standarduncertainty of the assigned value in theircalculation, will be reported in place ofz-scores.Q: How is z’- score (z-prime) calculated?A: A z’- score (z-prime) incorporates thestandard uncertainty of the assigned valueand is calculated as follows:z’ = (x - X)√ SDPA 2 2+ u xWhere x = participant resultX = the assigned valueSDPA = standard deviation forproficiency assessmentu x = standard uncertainty ofthe assigned value XQ: What is the estimated uncertainty of theassigned value?A: The assigned value has a standarduncertainty (u x ) that depends upon themethod used to derive the assigned value.When the assigned value is determined bythe consensus of participants’ results, theestimated standard uncertainty of theassigned value can be calculated by;u x = 1.25 x Robust standard deviation/√ nWhere n = number of resultsWhen the assigned value is determined byformulation, the standard uncertainty isestimated by the combination of uncertaintiesA z’-score is interpreted in exactly the sameway as a z-score, ≤2 is satisfactory, >2 but