Refers to widely differing estimates of the treatment effect (that is, heterogeneity or variability in results) across studies that suggest true differences in underlying treatment effect. When the magnitude of intervention effects differs, explanations may lie in the patients (e.g. disease severity), the interventions (e.g. doses, co-interventions, comparison interventions), the outcomes (e.g. duration of follow-up) or the study methods (e.g. randomized trials with higher and lower quality risk of bias).