Table 3.

Bradford-Hill Criteria to Consider When Identifying Control Outcomes or Exposures

Criteria	Description
Strength	The larger the association, the more likely that the association is causal. However, a small association does not mean that there is not a causal effect as expected treatment effects in medicine are often small.
Consistency	Findings have been replicated by other researchers and/or in different samples.
Specificity	The more specific an association between a factor and an effect is, the greater the probability of a causal relationship. Causation is likely if the association is identified under specific circumstances and that there is no other likely explanation.
Temporality	Cause precedes effect; if there is an expected delay between the cause and expected effect, then the effect must occur after that delay.
Biological Gradient	For exposures that follow a dose-response curve, greater exposure should generally lead to greater incidence of the effect. In some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence.
Plausibility	A plausible biological mechanism between cause and effect is helpful.
Coherence	Coherence between epidemiological and laboratory findings increases the likelihood of an effect. Results need to be interpreted in light of existing data and known facts of the natural history and disease biology.
Experiment	Reducing exposure to the risk factor reduces the likelihood of the outcome.
Analogy	Exposures with similar mechanisms of action may result in similar outcomes.