[Meta-analysis as a tool for gaining knowledge].

In these days, more than one clinical trial is mostly performed to evaluate a new treatment or therapeutic intervention. This necessitates a combined evaluation of their results. An integration of evidence from several trials is also helpful to determine the actual knowledge. These are the main goals of meta-analyses. Since the end of the 80s meta-analyses are widely used in clinical research. At the beginning of a meta-analysis, a protocol has to be developed. Similar to a protocol of a clinical trial, the inclusion and exclusion criteria for trials, the hypotheses and the planned analyses have to be fixed. After a careful localization of trials, a combined statistical analysis is performed. An investigation of heterogeneity, i.e., differences between study results, is indispensable. During the last years, the tool meta-analysis has been criticized. The criticism mainly results from poorly conducted meta-analyses which generated results without prespecifying hypotheses or which merely combined study results. Well-planned meta-analyses, on the contrary, have an increasing influence in clinical research.