Core concepts in statistics and research methods. Part 5: systematic review and meta-analysis

In the traditional hierarchy of evidence, meta-analyses of RCTs sit at the pinnacle (Fig. 1).However, there are issues with almost all meta-analyses that mean this position is not fully justified and that estimates from meta-analyses may be biased. 5One issue concerns the types of studies that can be included in a meta-analyses, which are often observational.