Given the limited variability in successful quitting predicted by smoking behavior and nicotine dependence, genetics studies are focusing increasingly on the smoking cessation phenotype. Retrospective cross-sectional measures of cessation, comparing former smokers to current smokers, have been used with some success (Breitling et al. 2009a). A more refined phenotype for smoking cessation is based on ability to quit among treatment-seeking smokers followed prospectively in longitudinal observational studies or clinical trials (Hughes et al. 2010). While retrospective measures tend to be less precise than longitudinal measures, they can be collected more efficiently on larger populations.
