Bradford Hill Criteria

The Bradford Hill criteria are a set of nine principles, formulated by Austin Bradford Hill in 1965, for evaluating whether an observed association between an exposure and a disease reflects a genuine causal relationship. Developed in the context of establishing that smoking causes lung cancer — against industry objections that correlation is not causation — the criteria provide a structured framework for causal inference in observational epidemiology.

The nine criteria are: strength of association, consistency, specificity, temporality, biological gradient (dose-response), biological plausibility, coherence, experiment, and analogy. Of these, only temporality is strictly necessary: causes must precede effects. The others are heuristic weights to be balanced against each other, not a checklist or algorithm. Hill himself was explicit that no mechanical procedure replaces scientific judgment about the totality of evidence.

The criteria predate the formal causal graph methods of Pearl's do-calculus and have been criticized for lacking mathematical precision; they remain, nonetheless, the dominant practical framework for causal reasoning in evidence-based medicine and public health policy. Their lasting contribution is not an algorithm but a discipline: insisting that the move from association to causation requires explicit argument rather than implicit assumption.