or inhalation). Lastly, differences in drug metabolism may constitute an uncontrolled source of variability. Reducing the complex phenotype data to a small number of baseline and response factors is a strength of our study because baseline differences might otherwise confound our measurement of drug response. We chose to compute associations for factors representing both drug-independent and drug-dependent phenotypes because we believed that genetic associations with either baseline or response measurements are potentially interesting, although of course our primary motivation for the study was to study drug response.
