Results of meta-analyses may differ from those of megatrials only because of the bias that could be introduced by a unadequate methodology of some meta-analyses. A meta-analysis must be based on an exhaustive review of the literature - of all studies, those with negative as well as positive results. The quality of a meta-analysis depends on the methodological quality of the studies it analyzes; accordingly they must be selected according to strict methodological inclusion and exclusion criteria, defined a priori. The value of a meta-analysis is that allows a treatment strategy to be assessed in populations more heterogeneous than those in clinical trials. Nonetheless the heterogeneity of protocols, of dosing, and of outcome measures can lead to bias of the treatment effect estimation. Different analytic techniques must be used to assess possible publication or selection bias (the so-called "funnel plot" method) and potential sources of heterogeneity (heterogeneity test, meta-regression, etc.). Meta-analyses of individual data make it possible to assess the treatment effect according to patient characteristics. Prospective meta-analyses or those planned before clinical trials can help to limit data heterogeneity by making study protocols less varied (treatment, follow-up, evaluation, etc.).