Meta-analysis: Principles and procedures

Meta-analysis should be as carefully planned as any other research project, with a detailed written protocol being prepared in advance.The a priori definition of eligibility criteria for studies to be included and a comprehensive search for such studies are central to high quality meta-analysis.The graphical display of results from individual studies on a common scale is an important intermediate step, which allows a visual examination of the degree of heterogeneity between studiesDifferent statistical methods exist for combining the data, but there is no single ''correct'' method.A thorough sensitivity analysis is essential to assess the robustness of combined estimates to different assumptions and inclusion criteria.