To clarify the relationship between CPD and FTCD-based nicotine dependence, we evaluated the correlation between total FTCD score and CPD. In a population sample of subjects from Missouri who had smoked at least 100 cigarettes, the correlation between FTCD and CPD was 0.81 in European Americans (n=11,312) and 0.71 in African Americans (n=3,031). The FTCD items related to early morning smoking had the lowest tetrachoric correlations with CPD both in European Americans (0.35) and African Americans (0.32): item 3: Which cigarette would you hate most to give up (1st AM cigarette), and item 5: Do you smoke more frequently during the first hours after waking than during the rest of the day. In a population sample of Missouri smokers, the correlation between CPD and early morning smoking was 0.69 in EA (n=11,286) and 0.58 in AA (n=3,028). FTCD has been previously described as a two-dimensional phenotype characterized by (1) CPD and (2) early morning smoking(32). The association between early morning smoking and rs1451240 is given in supplemental Table 2. These results suggest that studies using CPD as a phenotype may
