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Accreditation and Quality Assurance

, Volume 20, Issue 5, pp 347–353 | Cite as

Qualitative PT data analysis with easy-to-interpret scores

  • Steffen UhligEmail author
  • Christian Bläul
  • Kirstin Frost
  • Susann Sgorzaly
  • Bertrand Colson
  • Kirsten Simon
General Paper

Abstract

Numerical values called L-scores, with properties similar to those of z-scores, can be calculated for proficiency tests involving qualitative test methods. These scores do not require the use of replicates and are defined in such a way as to reflect both the Level of Competence of the Laboratory (LCL) and the Level of Difficulty of the Task (LDT). On the basis of the “logit” statistical model, the parameters corresponding to the LCL and the LDT values are determined via maximum likelihood estimation. With p denoting the corrected binary response, the model can be interpreted as \(\ln \left( {p/\left( {1 - p} \right)} \right) = LCL - LDT.\) The estimated parameters are then the quantities used to compute the L-scores for the participating laboratories. In the case of the laboratory-specific L-scores across tasks, the score for a particular laboratory can be computed as the deviation of the individual LCL from the average LCL relative to the standard error. The interpretation of the L-scores is slightly different from that of z-scores If |L| ≤ 2, the competence of the participant is not significantly different from the average competence. If L > 2, the competence is significantly higher, while if L < −2, the competence is significantly lower.

Keywords

Qualitative test method Proficiency test (PT) Binary data Rate of detection (RODProbability of detection (POD) z-score L-score Logit model 

Notes

Acknowledgments

The example is based on anonymized data from Joint Action QUANDHIP. The authors would like to thank the reviewers for their helpful comments.

Compliance with ethical standards

Conflict of interest

The authors declare that there are no conflicts of interest.

References

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Steffen Uhlig
    • 1
    Email author
  • Christian Bläul
    • 1
  • Kirstin Frost
    • 1
  • Susann Sgorzaly
    • 1
  • Bertrand Colson
    • 1
  • Kirsten Simon
    • 1
  1. 1.QuoData GmbHDresdenGermany

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