Heterogeneity: the issue of apples, oranges and fruit pie.

ABSTRACT

Heterogeneity refers to any kind of variation among studies contributing to the same outcome in a systematic review. There are three broad types of heterogeneity clinical heterogeneity, methodological heterogeneity and statistical heterogeneity. In this paper, we describe these three types of heterogeneity and the main statistical approaches to measure heterogeneity.