We specifically examined the relationship between FTCD and CPD to clarify the discrepancy between our FTCD-based results and CPD-based results. Specifically, FTCD includes measures for early morning smoking that are not well-captured by CPD. The difference between these phenotypes may be explained by the contrast in African Americans: although the odds ratios for the SNP using the FTCD-based definition of nicotine dependence are identical in European Americans and African Americans, the effect size for the regression onto CPD is subjectively smaller in African Americans than in European Americans(although not statistically significantly smaller).The inconsistent measurement of CPD as compared to FTCD has been previously described in the literature(21).
