A practical approach to Bland-Altman plots and variation coefficients for log transformed variables.
OBJECTIVE
Indicators of reproducibility for log-transformed variables can often not be calculated straightforwardly and are subsequently incorrectly interpreted.


RESULTS
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.


CONCLUSIONS
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.
Euser
Anne M
AM
Department of Clinical Epidemiology, Leiden University Medical Centre, The Netherlands.
Dekker
Friedo W
FW
le Cessie
Saskia
S
D003627:Data Interpretation, Statistical; D006801:Humans; D015588:Observer Variation; D015203:Reproducibility of Results; D012107:Research Design; D012885:Skinfold Thickness; D055815:Young Adult