The entire study sample of 534 smokers was pooled with an additional 577 individuals from the University of Pennsylvania Nicotine Replacement Trial (20) and 355 individuals from the Multi-Ethnic Cohort Diversity Panel (72) that included African-Americans, Asians, Native Hawaiians, Latinos, and Caucasians. These individuals provide reference ethnic groups and help to better estimate the admixture within a given individual. The program STRUCTURE was used to estimate the coefficient of ancestry for each individual (69) and principal component analysis was used to estimate the principal components for each individual (81). For the STRUCTURE analysis, the best fitting analysis estimated four ancestral populations within the combined sample: African, Caucasian, Asian, and Amerindian. The ancestral populations and estimated amounts of admixture within each individual corresponded well with the first four principal components—components that explained 39% of the variation with all the other components explaining 1% or less of the variation each. Visual inspection showed very little individual admixture within each self-identified ethnic group, indicating a small potential for confounding because of population stratification. In addition, we empirically tested the impact of population structure
