08/2-2009 COMMUTABILiTY

Commutability The hidden secrets By Dietmar Stöckl STT Consulting

Tin Can 3 - Commutability 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

Experiment and interpretation 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

Cholesterol example EP 14 +4% 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

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

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

Commutability of single, native samples +3.8% 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

Tin Can 3 - Commutability 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

Tin Can 3 - Commutability Calibrator set Tin Can 3 - Commutability

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