Blood or saliva samples were collected from participants in all four samples for DNA extraction and genotyping on a microarray chip. Each study employed a different genotyping platform, but used the genotyped polymorphisms to impute to a common set of variants in the 1000 Genomes Phase 1 reference panel (Abecasis et al., 2012) after applying standard quality control procedures. A summary of the methods for genotyping, quality control filtering, and imputation procedures across samples can be found in Table 1. All samples were assessed for population stratification using multidimensional scaling modeling, and only participants of European ancestry were included for analysis.
