When Bradford Hill's famous case-control study of the association between smoking and lung cancer was criticized, he listed the inferences used to establish causality,
Strength of association
Consistency
Specificity
Relationship in time
Biological gradient
Biological plausibility
Coherence of evidence
Experiment
Analogy
and said, "None of these nine viewpoints can bring indisputable evidence for or against a cause and effect hypothesis and, equally, none can be required as a sine qua non. What they can do, with greaten or less strength, is to help answer the fundamental questionis there any other way of explaining the set of facts before us, is there any other answer equally, or more, likely than cause and effect?