TABLE 6.

Examples of criteria to assess the risk of bias by study type¹

		Study type
Type of bias	Criterion	RCT	Cohort study	Case-control study	Cross-sectional study
Bias due to confounding	Were relevant confounding factors prespecified and considered?	NA	✓	✓	✓
	Were study groups balanced with respect to the distribution of confounding factors?	NA	✓	✓	✓
	Were confounding factors taken into account in the design and/or analyses?	NA	✓	✓	✓
	Was the assignment of participants to study groups randomized?	✓	NA	NA	NA
	Was an adequate method of concealment of allocation to study groups used?	✓	NA	NA	NA
Bias in selection of participants for the study	Were the same inclusion and exclusion criteria used for all study groups?	✓	✓	✓	✓
	Was the likelihood that some participants might have the outcome before the exposure or intervention assessed and taken into account in the design and/or analysis?	✓	✓	✓	✓
	Was the percentage of eligible nonparticipants in each study group below an acceptable value?	✓	✓	✓	✓
Bias in measurement of exposures and interventions	Was the exposure or intervention status measured in an accurate and sufficiently precise way?	✓	✓	✓	✓
Bias due to departures from intended exposures and interventions	Were there systematic differences between study groups in the care provided and/or in exposures to factors beyond those intended by study design?	✓	✓	✓	✓
	Was the exposure or intervention status assessed more than once or in >1 way to help ensure fidelity to the study design?	✓	✓	✓	✓
Bias due to missing data	Was the percentage of participants dropping out in each study group below an acceptable value?	✓	✓	✓	✓
	Were missing data appropriately handled (e.g., intention-to-treat analysis, imputation)?	✓	✓	✓	✓
Bias in measurement of outcomes	Were all relevant outcomes measured in an appropriately accurate and sufficiently precise way (e.g., valid and reliable) and done consistently across all study participants?	✓	✓	✓	✓
	Was the length of follow-up among study groups in prospective studies the same, or in case-control studies were the times between exposures or interventions and targeted outcomes the same in cases and controls?	✓	✓	✓	✓
	Was the assessment of outcome made “blind” to exposure or intervention status or, when blinding was not possible, was there recognition that knowledge of exposure or intervention status could have influenced the assessment of the outcome or outcomes?	✓	✓	✓	✓
Bias in selection of the reported result	Were the prespecified outcomes partially reported or not reported because of the statistical significance or magnitude of the effect of the exposure or intervention?	✓	✓	✓	✓
	Is there evidence that the results from all participants, not only a subset, were analyzed or that all multiple-adjusted analyses, not only selected ones, were fully reported?	✓	✓	✓	✓

¹NA, not applicable; RCT, randomized controlled trial; ✓, applicable to the study type.