Ideally, all GWAS research in a specific field should be conducted with the upfront aim that the data generated from different teams will be combined prospectively as they accumulate. The development of consortia of multiple teams can facilitate this process [10]. Consortia have been developed in many disease fields and they may cover a large number of teams [11]. In some diseases, several non-overlapping consortia exist, each one comprised of multiple teams of investigators, while other teams may not participate in any consortium, but may nevertheless conduct their own GWA and/or replication studies. Moreover, most GWA datasets are generated in epidemiological studies that have been set up in the past, and many important aspects of their design, conduct, data and sample collection can no longer be altered. Therefore, certain limitations that arise from retrospective features are almost unavoidable, even for the most inclusive, international, prospective effort.
