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Table 5 Examples of study-level risk of bias classifications

From: Principles and framework for assessing the risk of bias for studies included in comparative quantitative environmental systematic reviews

Risk of bias classification for each class of bias Possible study-level risk of bias classification for a given outcome of interest
All classes of bias are judged to have low risk of bias, definitely low risk of bias, or probably low risk of bias for the outcome of interest Low risk
At least one class of bias is judged to have high risk of bias, definitely high risk of bias, or probably high risk of bias for the outcome of interest High risk
Some classes of bias are judged to have low risk, others unclear risk, but no classes are judged to have high risk for the outcome of interest Unclear risk (to avoid an unclear judgement for the overall study it is preferable for each domain to reach a probably low risk or probably high risk judgement instead of an “unclear” judgement, where possible)
No information available for any classes of bias for the outcome of interest Unclear risk (or no information)
Classes of bias are judged as having combinations of “moderate”, “serious” or “critical” risks of bias (or other terminology) for the outcome of interest Summarising judgements other than high/low/unclear may not be intuitively straightforward. A clear rationale should be provided, based on logic (i.e. the criteria should not be arbitrary). See recent Cochrane tools [58, 59, 81] for examples