4 principal components (PCs) (for details and PC plots, see the article by Nelson et al 27). For analyses that divided the neighborhood controls on the basis of lifetime licit and illicit drug dependence, a separate PCA run for each comparison found in each instance a single significant PC with the following P values: cases versus neighborhood controls not dependent on nicotine, alcohol, and illicit drugs, P = .06; cases versus neighborhood controls not dependent on alcohol and illicit drugs, P = .06; cases versus neighborhood controls not dependent on illicit drugs, P = .02; cases versus illicit drug-dependent neighborhood controls, P = .02; illicit drug-dependent versus non-dependent neighborhood controls, P = .001. Each of these analyses included a single PC as a covariate to control for admixture.
