Two genome-wide association analyses were conducted in PLINK (25). The first used logistic regression with nicotine dependence as the dependent variable, and the second used linear regression with CPD as the dependent variable. Genotypes were coded log-additively (0, 1, 2 copies of the minor allele). Covariates representing sex, age (defined, using quartiles, as 3 indicator variables representing 34 years and younger (reference), 35–39 years, 40–44 years, and 45 years and older), self-reported ethnicity, and alcohol and cocaine dependence (the diagnoses used to ascertain subjects for the original COGA and FSCD studies) were included.
