Table 1. Guidelines for causal inference.

U. S. Surgeon General report’s criteria	Hill’s criteria
Consistency of association	Strength
Strength of association	Consistency
Specificity of association	Specificity
Temporal relationship of association	Temporality
Coherence of association	Biological gradient
	Coherence
	Experiment

Sources: Smoking and health: Report of the Advisory Committee to the Surgeon General, 1964 (69) and Hill AB, 1965 (33)