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 studies Different 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.