We included all subjects who had ever smoked a cigarette. Cases were defined with a Fagerström Test for Cigarette Dependence (FTCD) score of 4 or more, and controls had an FTCD score ≤ 3, based on the dichotomous nicotine dependence phenotype used for COGEND. For our case definition, we also included 100 individuals who smoked on average more than a pack a day, but had a missing FTCD score. This is consistent with previous research that found that most individuals who smoke more than 20 cigarettes a day have an FTCD score of 4 or greater(9). The final sample for association testing contains 1,294 nicotine dependent cases and 2,071 non-dependent controls who have smoked at least one cigarette (Table 1).
