[Statistical Methods for Comparison of Two Measuring Procedures and for Calibration: Analysis of Concordance, Correlation and Regression in the Case of Measuring Intraocular Pressure]

Background: The comparison of two or more quantitative measurement procedures applied to the same patient is often carried out by unsuitable statistical methods. An orientation for the choice of methods has been given for such problems.

Methods: Graphical methods, the concordance correlation coefficient and some regression straight lines are presented for the judgement of concordance of measurement values, and they are exemplary explained for the measurement of the intraocular pressure with lid tonometry and the ocular blood flow system (OBF).

Results: The t-test for paired samples and tests of equivalence are invalid in this context. The Pearson correlation coefficient measures to what degree two variables are associated over a linear relation with reference to the population. It ignores location shift as well as scale shift, and it says nothing about the intraindividual concordance. A suitable procedure is the concordance correlation coefficient which can decomposed into precision, location shift, and scale shift.

Conclusions: An assessment of concordance of intraindividual measurement values should include an x-y-graphic of the measured points, a Bland-Altman plot, and the concordance correlation coefficient.