Table 1.

Bradford Hill's original guidelines and proposed revisions

Type of evidence	Revised, structured guidelines	Hill's original guidelines
Direct	Size of effect not attributable to plausible confounding	Experiment
	Appropriate temporal and/or spatial proximity (cause precedes effect and effect occurs after a plausible interval; cause occurs at the same site as the intervention)	Strength
	Dose‐responsiveness and reversibility	Temporality
Mechanistic	Evidence for a mechanism of action (biological, chemical, mechanical)	Biological gradientBiological plausibility
Parallel	Coherence	Coherence
	Replicability	Consistency
	Similarity	Analogy