Table 1

Bradford Hill criteria of causality and their relation to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) criteria for upgrading and downgrading

Bradford Hill criteriaConsideration in GRADE
StrengthStrength of association and imprecision in effect estimate
ConsistencyConsistency across studies, ie, across different situations (different researchers)
TemporalityStudy design, specific study limitations; RCTs fulfil this criterion better than observational studies, properly designed and conducted observational studies
Biological gradientDose–response gradient
SpecificityIndirectness
Biological plausibilityIndirectness
CoherenceIndirectness
ExperimentStudy design, randomisation, properly designed and conducted observational studies
AnalogyExisting association for critical outcomes will lead to not downgrading the quality, indirectness