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Systematic Reviews: Assessing Risk of Bias


Bias may result from systematic errors in the research methodology.

Common types of bias:

Selection bias

- Only choose studies which they are familiar with or support their view-point

- Occurs when the two groups being compared differ systematically, eg. differences in the characteristics between those who are   selected for a study. 

Performance biasdifferences between groups in the care that is provided, or in exposure to factors other than the interventions of interest.

Detection biasdifferences between groups in how outcomes are determined. 

Attrition biasdifferences between groups in withdrawals from a study.

Reporting biasdifferences between reported and unreported findings.

Publication bias - Only select studies which have been published. In general, studies with statistically significant or positive results are more likely to be published than those with non-significant or negative results.(1)

Language bias - Only select studies written in English

Indexing bias - Only search a limited number of databases or use a limited choice of search terms.



1. Song F, Parekh S, Hooper L, et al. Dissemination and publication of research findings: an updated review of related biases. Health Technol Assess. 2010;14(8):iii, ix–xi, 1–193. doi: 10.3310/hta14080

Assessing Risk of Bia