In a systematic review, the variability of or differences in the selected studies.

Note: A distinction is sometimes made between “statistical heterogeneity” (differences in the reported effects) and “methodological heterogeneity” (differences in study design with regard to the key characteristics of the subjects, interventions or outcome evaluation criteria). Statistical tests of heterogeneity are used to determine whether the observed variability in study results effect size is greater than the variability that can be expected to occur by chance. However, these tests have low statistical power.

Antonym: homogeneity.