Evaluation of Proficiency Test Data by Different Statistical Methods Comparison

In analytical chemistry, proficiency testing usually consists in tests that laboratories conduct under routine conditions and report the result to PT provider who then converts the result to a score which helps the participant to assess the accuracy of theresult. The aim of this work is to show PT providers, accreditations bodies and participating laboratories that different scoring results can be achieved depending on the selected evaluation system. The influence of different evaluation techniques on the results of an interlaboratory comparison for the determination of gold in precious metals alloys was investigated. The results of 19 participating laboratories were evaluated by means of the following procedures: 1.ISO 5725 / Harmonized Protocol / ISO Guide 43-1 2.Robust Methods:2.A. Robust procedure; 2.B. ISO 13528 3.Fitness for purpose The evaluation of the same PT data showed very interesting issues depending on the different scoring systems that were used as well as robustness of statistical methods for detecting outliers. As a general rule, laboratories with scoring Z > 2 offered clearly poorer performance in robust approaches than classical ones. The selection and comparison of different scoring systems have to be done very carefully because sometimes they are not the best approach for studying the data population or the more appropriate one for evaluating the distribution of the data. Finally it should be taking into account that sometimes the robust scoring systems are not always suitable for evaluating the results of certain PT schemes.