Invited commentary: consolidating data harmonization--how to obtain quality and applicability?

It is recognized that very large sample sizes capable of providing adequate statistical power are required to properly investigate and understand the role and interaction of genetic, lifestyle, and environmental factors in modulating the risk and progression of chronic diseases. However, very few one-off studies provide access to very large numbers of participants, and the collection of high-quality data necessitates a major investment of resources. The scientific community is thus increasingly engaged in collaborative efforts to facilitate harmonization and synthesis of data across studies. Complementary harmonization approaches may be adopted to support these efforts. In the current issue of the American Journal of Epidemiology, Hamilton et al. (Am J Epidemiol. 2011;174(3):253-260) present the consensus measures for Phenotypes and eXposures (PhenX) Toolkit, which promotes the use of identical data collection tools and procedures to support harmonization across emerging studies. Data synthesis is greatly facilitated by the use of common measures and procedures. However, the "stringent" criteria required by PhenX can limit its utilization. The opportunity to make use of rigorous but more "flexible" harmonization approaches should also be considered.