There are two primary differences between the FTCD definition of nicotine dependence and CPD: (1) nicotine dependence is based on a 10 point FTCD scale computed from 6 items including CPD, and (2) nicotine dependence is a dichotomous variable whereas CPD is an ordinal variable. Therefore we created four phenotypes for evaluation: dichotomous nicotine dependence, dichotomous CPD, ordinal FTCD score, ordinal CPD. Using each of these phenotypes, we looked at the association with rs1451240 in multiple stratifications of the data: gender, ethnicity, age, and comorbid substance dependence (Table 3).
