a trait-by-trait basis. [45–48] Other consortia and projects such as the Public Population Project in Genomics (http://www.p3gconsortium.org/) and PhenX (www.phenx.org) aim to facilitate broad collaboration among existing and future genome-wide association studies by making recommendations for standard phenotyping protocols for many diseases and traits. Still, despite best efforts to harmonize measures, some measurement differences across studies will persist, and investigators should be aware of these as possible sources of heterogeneity (see Sections 4 and 5).
