[Systematic reviews in five steps: IV. Summarizing the evidence]

This fourth in a series of five articles on systematic reviews demonstrates how to compile and summarize the results of the studies included in a review. The synthesis of the extracted data consists of tabulating the characteristics, and the quality of the included studies, with the effects or the magnitude of the associations found in them. Statistical methods can be used to explore the differences between studies and the reasons for the inconsistencies. The magnitude of this heterogeneity influences whether it is feasible to perform an overall or subgroup meta-analysis. Finally, information is provided both to assessment of clinical and methodological reasons for heterogeneity.