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08/2-2009

08/2-2009. COMMUTABILiTY. Commutability The hidden secrets. By Dietmar Stöckl ST T Consulting. Definition. Commutability of a reference material

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08/2-2009

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  1. 08/2-2009 COMMUTABILiTY

  2. Commutability The hidden secrets By Dietmar Stöckl STT Consulting

  3. Definition • Commutability of a reference material • property of a reference material, demonstrated by the closeness of agreement between the relation among the measurement results for a stated quantity in this material, obtained according to two given measurement procedures, and the relation obtained among the measurement results for other specified materials • NOTE 1 The reference material in question is usually a calibrator and the other specified materials are usually routine samples (can also be trueness controls, DS) • NOTE 2 The measurement procedures referred to in the definition are the one preceding and the one following the reference material (calibrator) in question in a calibration hierarchy (see ISO 17511) (in general, can be any two measurement procedures, DS). • JCGM 200:2008. International vocabulary of metrology – Basic and general concepts and associated terms (VIM). International Bureau of Weights and Measures (BIPM); Joint Committee for Guides in Metrology (JCGM): Paris, 2008. Freely available at: http://www.bipm.org/en/publications/guides/vim.html Tin Can 3 - Commutability

  4. Experiment and interpretation • Experiment • Measure a minimum of 20 native samples and test samples (trueness controls, calibrators) in one run in replicate (EP 14, n = 3) with at least two measurement procedures • General interpretation • “Compare the behaviour of the test samples with the native samples” • EP 14 interpretation • Test samples should be within the prediction limits of the regression based on the native samples Tin Can 3 - Commutability

  5. +4% Cholesterol example EP 14 • The test sample is just at the prediction limit and is considered commutable. When this sample is used as trueness control in proficiency testing, the results are flawed by 4%! Tin Can 3 - Commutability

  6. +0.8% Different point of view • Commutability should be judged versus the confidence interval of the regression line! Tin Can 3 - Commutability

  7. Commutability – A different view • Samples with confidence intervals versus CI of regression line. Tin Can 3 - Commutability

  8. +3.8% Commutability of single, native samples • Single, native samples may have sample-related effects and may not represent the average sample! When this sample is used as trueness control in PT, the results are flawed by 3.8%! Tin Can 3 - Commutability

  9. Related samples • CAVE! The statistical treatment of related samples (dilutions of calibrators; PT materials with the same matrix) has not been addressed, yet. Tin Can 3 - Commutability

  10. Calibrator set Tin Can 3 - Commutability

  11. Commutability – Current concept • The statistical concept is wrong • It is about the confidence interval of the line and not about the prediction interval. We want to assess the trueness of the measurement procedure or we want to calibrate the procedure • Unsolved problem • Related samples • Remark • Not merely a statistical problem! Check the quality of the input data! Tin Can 3 - Commutability

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