Although CPD is the most common phenotypic measurement of smoking behavior, there is strong epidemiological evidence that the number of cigarettes smoked per day varies across cultures and ethnicities. For example, African Americans smoke fewer cigarettes than European Americans(21). However, the FTCD score, ranging from 0 to 10 where CPD can account for a maximum of four levels, defined as 1–10, 11–20, 21–30, or 31 or more, appears to be an invariant measure of nicotine dependence across ethnicities(21). Therefore, we hypothesized that a genome-wide association study with FTCD rather than CPD may have increased power to detect variants associated with nicotine dependence, especially in a multi-ethnic sample.
