Medicine Subject Guides: Risk of Bias

In systematic reviews, the risk of bias refers to the potential for systematic errors or flaws in the design, conduct, or reporting of individual studies included in the review. Assessing the risk of bias helps evaluate the internal validity of the studies and the overall confidence in the review findings. Here are some common sources of risk of bias in systematic reviews:

Selection Bias: This occurs when there is a systematic difference between the characteristics of the participants selected for a study and the target population. It can arise from inadequate randomization, non-representative sampling, or exclusions of certain groups.

Performance Bias: Performance bias refers to systematic differences in the care provided to participants in different study groups. It can arise from factors such as lack of blinding, differences in adherence to treatment protocols, or variations in co-interventions.

Detection Bias: Detection bias occurs when there are systematic differences in outcome assessment between study groups. It can arise from lack of blinding of outcome assessors, differential follow-up rates, or variations in the use of validated measurement tools.

Attrition Bias: Attrition bias, also known as dropout bias, occurs when there are differential rates of participant withdrawal or loss to follow-up between study groups. It can introduce bias if the reasons for attrition are related to the outcomes being measured.

Reporting Bias: Reporting bias refers to the selective reporting of study outcomes based on their statistical significance or the direction of results. It can occur when studies only report positive or statistically significant findings, while omitting negative or non-significant results.

Publication Bias: Publication bias arises from the selective publication of studies based on their results. Studies with positive or statistically significant findings are more likely to be published, while studies with negative or non-significant findings may remain unpublished. This can lead to an overestimation of treatment effects.

Other Sources of Bias: There can be additional sources of bias specific to certain study designs or research areas. For example, in diagnostic accuracy studies, incorporation bias or index test review bias can occur.

RoB 2 — Cochrane risk-of-bias tool for randomized trials

RoBiS — Risk of Bias tools for Systematic Reviews developed by University of Bristol

CASP(Critical Appraisal Skills Programme) — checklists for systematic reviews, RCTs, qualitative studies etc.

Newcastle-Ottawa Scale (NOS): The NOS is commonly used for assessing the risk of bias in non-randomized studies, including cohort studies and case-control studies. It evaluates the quality of studies based on three main domains: selection of study groups, comparability of groups, and ascertainment of outcomes.

After assessing the risk of bias, you may generate a risk of bias table to show the quality of each literature. 

Robvis (Risk of bias visualization tool) is a web app designed to for visualizing risk-of-bias assessments performed as part of a systematic review.