A practical approach to Bland-Altman plots and variation coefficients for log transformed variables.

Indicators of reproducibility for log-transformed variables can often not be calculated straightforwardly and are subsequently incorrectly interpreted. We discuss meaningful Coefficients of Variation (CV) for log-transformed variables, which can be derived directly from the standard error of the log-transformed measurements. To provide easy interpretable Bland and Altman plots, we calculated limits of inter and intraobserver agreement (LA) for log-transformed variables and transform them back to the original scale. These LAs for agreement are subsequently plotted on the original scale in a conventional Bland and Altman plot. Both approaches were illustrated in a clinical example on the reproducibility of skinfold thickness measurements. In reproducibility, it is important to calculate meaningful CVs, LAs, and Bland-Altman plots for log-transformed variables. We provide a practical approach in which existing statistical methods are applied in the field of reproducibility, thus leading to parameters of reproducibility which can be interpreted on the original scale.