Commentary: Heterogeneity in meta-analysis should be expected and appropriately quantified

It is generally accepted that meta-analyses should assess heterogeneity, which may be defined as the presence of variation in true effect sizes underlying the different studies. This assessment might be achieved by performing a statistical test for heterogeneity, by quantifying its magnitude, by quantifying its impact or by a combination of these. Patsopoulos, Evangelou and Ioannidis propose methods for examining the effect of excluding studies (or groups of studies) on an assessment of heterogeneity. 1 Their methods offer benefits over the sometimes practiced ‘leave one out’ approach to sensitivity analysis, by recognizing that the overall effect (against which heterogeneity is measured) changes each time an…