We used the software package EIGENSTRAT (24)to calculate principal components reflecting continuous variation in allele frequencies, representing ancestral differences in subjects. Two principal components were identified. The first distinguished African American participants from European American participants, and the second distinguished Hispanic from non-Hispanic subjects. These scores were included to control for effects of population stratification. In addition, we used self-reported ethnicity (European American, African American or Hispanic) as a categorical variable and compared results with those using the first two principal components.
