# MedStat Stutter

## Phantasies of a physicist fallen among physicians

### Pairwise differences in cross-over design

November 8, 2016, 11:13 am

A colleague asked me how to present the results of a study in gastric emptying. As you have seen multiple time here, the emptying time t50 is the target variable. But read on even if you work with blood pressure (also often misused as example), rehabilitation, or pharmacology. The methods are the sa...

The coefficients of linexp fits to gastric emptying curves are generally used in mixed-models to analyze group and meal difference. The distribution of the extracted parameters is often highly skewed, so before using linear models or estimates of reference ranges with functions in package referenceInterval or Hmisc, a transformation might be required.
From a large corpus of gastric emptying curves analyzed with nonlinear fits of the linexp function, the following recommendations for transformation were obtained using the boxcox and the logtrans function in package MASS: