Clinical diversity (or heterogeneity) is the variability between the patients or the interventions being studied, or between the outcomes that the studies measure. When comparing different studies, it is important to bear in mind that there are several types of heterogeneity.

Methodological diversity (or heterogeneity) refers to variation in study design, and in the risk of bias between studies.

Clinical and/or methodological diversity can lead to differences in the way statistics are applied to different studies (statistical heterogeneity).

Progress in medical science is improving our understanding of heterogeneity among patients with the same disease. The differences in patient responses to treatment, and the risk of adverse reactions, are being explored at the molecular level. This is leading to the development of targeted treatments for different subgroups of patients.