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Note on the bimodal distribution of PT results

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Abstract

The practitioner’s report "Treatment of bimodality in proficiency test of pH in bioethanol matrix" by Sarmanho et al. deals with a proficiency test (PT) for the measurement of pH in bioethanol. One group of 19 participating laboratories used an electrode that contained saturated LiCl and the other one KCl. The results of the LiCl group are biased, and those of the KCl group are unbiased as compared with the assigned reference value \(y_\mathrm{{RM}}\), and hence, the distribution of the laboratory means is bimodal. In order to deal with this situation, Sarmanho et al. present and carry out two different methods of fitting the bimodal distribution of the laboratory means: a fit by a mixture of two normal distributions and a Gaussian kernel estimation. Since this rather advanced statistical analysis does not help to solve the practical problem, some recomendations of ISO 17043 are discussed that might be useful for the coordinator of the PT and as well for the participating laboratories.

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References

  1. Sarmanho GF, Borges PP, Fraga ICS, Leal LHDC (2015) Treatment of bimodality in proficiency test of pH in bioethanol matrix. Accred Qual Assur 20:179–187

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  2. ISO/IEC 17043 (2010) Conformity assessment–general requirements for proficiency testing. International Organization for Standardization, Geneva

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  3. ISO 5725–2 (1994) Accuracy (trueness and precision) of measurement methods and results–part 2: basic method for the determination of repeatability and reproducibility of a standard measurement method. International Organization for Standardization, Geneva

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  4. ISO 13528 (2015) Statistical methods for use in proficiency testing by interlaboratory comparison. International Organization for Standardization, Geneva

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Authors and Affiliations

  1. Institut für Statistik und Ökonometrie, Freie Universität Berlin, Garystrasse 21, 14195, Berlin, Germany

    Peter-Th. Wilrich

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  1. Peter-Th. Wilrich
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Correspondence to Peter-Th. Wilrich.

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Wilrich, PT. Note on the bimodal distribution of PT results. Accred Qual Assur 21, 381–383 (2016). https://doi.org/10.1007/s00769-016-1229-5

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  • DOI: https://doi.org/10.1007/s00769-016-1229-5

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