environmental exposure data. The participating studies vary widely in design: some of the longitudinal studies have been ongoing for years or even decades, others have international data collection sites, and some are family-based studies with phenotype and genotype data available on multiple family members. Using GENEVA as a practical example, this paper provides an illustration to guide GWAS consortia through the process of phenotype harmonization and describes key phenotype harmonization issues that arise when sharing data across disparate studies. The principles outlined here can also be applied to other data sharing contexts.
