Research Techniques Made Simple: Assessing Risk of Bias in Systematic Reviews

Systematic reviews are increasingly utilized in the medical literature to summarize available evidence on a research question. Like other studies, systematic reviews are at risk for bias from a number of sources. A systematic review should be based on a formal protocol developed and made publicly available before the conduct of the review; deviations from a protocol with selective presentation of data can result in reporting bias. Evidence selection bias occurs when a systematic review does not identify all available data on a topic. This can arise from publication bias, where data from statistically significant studies are more likely to be published than those that are not statistically significant. Systematic reviews are also susceptible to bias that arises in any of the included primary studies, each of which needs to be critically appraised. Finally, competing interests can lead to bias in favor of a particular intervention. Awareness of these sources of bias is important for authors and consumers of the scientific literature as they conduct and read systematic reviews and incorporate their findings into clinical practice and policy making.