We defined two study samples within COGA. The first sample included all participants of European ancestry 25 years of age or older with both genome-wide association data and relevant SUD phenotypic information (n = 5582 individuals from 1093 extended families; 3009 (54%) female; Mage = 42.29 years, age range = 25 – 91 years). We limited the sample to those of European ancestry to avoid population stratification [32] because the educational attainment genome-wide association study (GWAS) weights come from a European ancestry discovery sample. SNPrelate [33] was implemented to estimate principal components from GWAS data and subsequently used to determine European ancestry. We implemented the age minimum to balance the needs to ensure that the majority of participants had passed through the period of highest risk for onset of the SUDs without unduly limiting sample size. Epidemiological data regarding age of onset for SUDs [34–36] guided our decision to select age 25 as the cutoff, which also mirrors the cutoff used in analyses of educational attainment in US Census data [37].
