A systematic literature review (SLR) is considered the highest form of evidence due to its rigorous approach through which every relevant piece of published or unpublished literature that is currently available to address a specific research issue is pooled and analyzed to answer a research question. For this reason, it is essential that the risk of bias (ROB) is assessed for all studies that are included in the SLRs. ROB assessment ensures the transparency of the outcomes, the validity of evidence, confidence, and reproducibility of the SLR process. It is typically done by identifying systematic errors or limitations in the conduct, design, or analysis of each included study in SR. Further, ROB assessment of the included studies is also a requirement for optimal reporting of SLRs, recommended by the PRISMA 2020 statement. (1,2)
The tool for ROB assessment depends on the study design of the included studies. For SLRs including RCTs, different tools are available for ROB assessment; one such tool is the Cochrane risk of bias tool version 2.0 (RoB-2), that was published in 2019 as an upgrade to the previous Cochrane RoB tool. According to the developers, the Cochrane RoB-2 is suitable for assessing ROB in individually-randomized, parallel-group, and cluster- randomized trials. (3) Other tools that are used for assessing ROB of RCTs include the EPOC RoB Tool for complex interventions randomized trials, the Critical Appraisal Skills Programme (CASP) checklist, the Joanna Briggs Institute (JBI) critical appraisal checklist, and the Scottish Intercollegiate Guidelines Network (SIGN) critical appraisal checklists for assessing methodological quality of different study types, including RCT. (4-6) Out of all of these available tools, the choice of the most appropriate tools depends on the research question, domain coverage of the tool, availability of the tool, the type of scoring system used, and any specific regulatory requirement.
For SLRs including non-randomized studies, a frequently used tool is the ROBINS-I (Risk Of Bias In Non-randomized Studies – of Interventions) tool. Apart from this, other popular tools include the JBI critical appraisal checklist for non-randomized experimental studies, the EPOC RoB tool, and the methodological index for non-randomized studies (MINORS) tool. (1,5,6)
There are several available tools for observational studies. To name a few, the CASP cohort study checklist, the SIGN critical appraisal checklists, the NIH quality assessment tool, the Newcastle-Ottawa Scale, and JBI critical appraisal checklist are the recommended tools for cohort study and Case-control studies. The Appraisal tool for Cross-Sectional Studies (AXIS) is another recommended tool for cross-sectional studies. Additionally, MINORS and the JBI Critical Appraisal tool can be used for ROB assessment of case series and case reports. (4-7) If an SLR includes more than one type of study design, the ROB assessment must use multiple tools based on the study design. Alternatively, a recently developed mixed methods appraisal tool (MMAT) can be used in such a situation.(8) Finally, for SLRs of SLRs, tools such as ROBINS and AMSTAR-2 are available, which can assess the risk of bias or methodological quality of the included SLR.(9,10)
While assessing ROB of the included studies in an SLR, it is essential to ensure that the latest version of the appropriate tool is used, and that the tool is validated and reliable. It is also recommended that independent assessment of ROB is carried out, and the results consolidated so that a comprehensive ROB assessment is performed. Finally, the reporting of ROB of the included studies must follow the recommendations of the tool.
While systematic reviews are the gold standard of evidence synthesis, their findings highly depend on the validity of their assessed studies. Therefore, the researchers must ensure that these studies are reliable and free from bias, leading to more accurate conclusions and evidence-based recommendations.
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