Types of Bias in
Systematic Reviews

In scientific research, a bias is a systematic or methodological error that causes misrepresentation of the study’s outcomes. The possibility of unfair or misleading interpretation of evidence outcomes in a systematic review can have serious implications; especially, in fields such as evidence-based medicine, pharmacovigilance, or while recommending guidelines for clinical practice.

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A bias can be introduced in a study at any stage of the process – from formulating the research question, establishing the eligibility criteria for inclusion and exclusion of primary studies, reviewing collected resources, to choosing which findings to publish. The hallmark of a systematic review is a reduced risk of bias. However, they are not fully immune to bias. The strengths and weaknesses of a systematic review depend solely on how the reviewer addresses the introduced errors. Let us look at the type of biases that can creep into a review in each of its stages.

Bias In Study Design

This kind of bias arises in the first step of formulating the review design and protocol. It could introduce a bias in the way the author frames the research question due to insufficient knowledge in the field of research. The author could, for example, decide to include only males in the study, assuming that no previous studies have been conducted on females. Other errors may arise due to an inefficient search strategy. For example, if reviewers have assigned arbitrary search limiters such as geographical regions or year of publication. Imposing such limiters will undoubtedly produce a biased sample set since it fails to collect all the available evidence.

Selection Bias

This kind of bias is introduced while collecting the primary resources for the study. If the collection of resources is not exhaustive, it could lead to over or underestimation of the results. The collection of resources for a systematic review must include all available resources, including grey literature. Potential personal bias can also be introduced by the reviewers in charge of selecting the primary studies. Key concepts regarding the eligibility criteria of studies included and excluded in the review must be clearly stated to avoid this kind of bias. Most of the known errors in systematic reviews arise in the selection and publication stages.

Publication Bias

An author or publisher may not publish a study whose results are negative or are not statistically significant. This is called publication bias. The outcomes may not be of relevance to the publisher but may have serious clinical implications.

Selective Outcome Reporting

Reporting errors or bias is a major threat to a systematic review. The author or reviewer may decide to only report a selection of outcomes that suit his or her interest.