For many traits, phenotype definitions may vary considerably across studies, or there may be many different options for defining the phenotypes of interest within each study. Some of this variability is unavoidable and results from differences in measurement protocols across studies. For example, disease may be self-reported in some studies or clinician-diagnosed in others; waist:hip ratios may be self-reported or measured in a clinic, using different operational definitions of “waist;” etc. Characteristics of studied phenotype may also differ across studies: for example, because of the widespread use of Prostate-Specific Antigen (PSA) screening in the United States since the early 1990s, the proportion of early-stage prostate cancer cases in the U.S. is higher than in Europe, where PSA screening is not as common. In the context of a prospective meta-analysis, study investigators can discuss these issues and reach consensus on how to define phenotype so as to maximize relevant information while ensuring as many studies can provide data as possible. In general, there is a trade off between more accurate (but more expensive and perhaps more invasive) measurements on fewer people
