The Implementation of a Statistical Evaluation Model"z-Score Evaluation-Risk Assessment-Technical Verification"for Proficiency Testing with Low Sample Size:Taking Water Quality Monitoring as an Example
In this paper,a statistical evaluation model of"z-score evaluation-risk assessment-technology verification"was established to reduce the deviation caused by low sample size when using z-score for the evaluation of proficiency testing from the true level of participants'results.Taking an example of the international proficiency testing project of"Iron and Fluoride Detection in Water"for the countries along the Belt and Road,the developed model was further verified.The results showed that when the number of testing results was only 14(low sample size)and the absolute value of the relative error between the assigned value and the standard value of the testing sample was over 4%,five participants within 14 participants who had obtained satisfactory evaluations on the iron item were at medium/high risk of inaccurate.Among 11 participants who had received satisfactory evaluation on the fluoride item,7 participants were at medium/high risk of inaccurate.The typical problems revealed by the technical verification were that the quality control process was implemented without following the requirements of detection methods,and the records of standard curves,reagent preparation,and key parameters of instrument were lack.It was hard for the traceability of testing records.Both the existence of risk when z-score evaluation used for proficiency testing with low sample size and the reliability and necessity of the statistical evaluation model was further verified.
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