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Hill's Criteria and Authoritative Citations

1) Strength of an association is a clue to causation, although a strong association is neither necessary nor sufficient to affirm causality, and a weak one is neither necessary nor sufficient to deny causality.

National Cancer Institute
- “In epidemiologic research, relative risks of less than 2 are considered small and usually difficult to interpret. Such increases may be due to chance, statistical bias or effects of confounding factors that are sometimes not evident.” – National Cancer Institute, “Abortion and possible risk for breast cancer: analysis and inconsistencies,” October 26, 1994.

Sir Richard Doll - " ... when relative risk lies between 1 and 2 ... problems of interpretation may become acute, and it may be extremely difficult to disentangle the various contributions of biased information, confounding of two or more factors, and cause and effect."
“The Causes of Cancer," by Richard Doll, F.R.S. and Richard Peto.  Oxford-New York, Oxford University Press, 1981, p. 1219.


WHO/IARC - “Relative risks of less than 2.0 may readily reflect some unperceived bias or confounding factor, those over 5.0 are unlikely to do so.”  - Breslow and Day, 1980, Statistical methods in cancer research, Vol. 1, The analysis of case control studies. Published by the World Health Organization, International Agency for Research on Cancer, Sci. Pub. No. 32, Lyon, p. 36

FDA - “Relative risks of 2 have a history of unreliability” - Robert Temple, M.D. Food and Drug Administration Journal of the American Medical Association (JAMA), Letters, September 8, 1999

FDA - "My basic rule is if the relative risk isn't at least 3 or 4, forget it." - Robert Temple, director of drug evaluation at the Food and Drug Administration.


Average cancer risk elevation for exposure to passive smoke: about 20% (relative risk=1.2)
Average cardiovascular disease risk elevation for exposure to passive smoking: about 30% (relative risk=1.3)

2) Consistency of results from different studies is an obvious attribute of true causal relationships. 3) Specificity requires that a cause leads to a single effect, which is seldom the case in multi-factorial epidemiology.   4) Temporality. That effects must occur after the cause has a chance to act is a self-evident and trivial criterion of causality.   5) Dose-effect relationship is a useful criterion of causation, but does not resolve the matter. 6) Plausibility. Whether an association is biologically plausible or not remains a matter of individual speculation and is far from being objective or conclusive. 7) Coherence. Agreement with other information may be a corollary attribute but not evidence of causation. 8) Experimental evidence. Experimental evidence in humans would indeed constitute proof of causation, but it is unavailable in the case of passive smoke. 9) Analogy is open to imagination and remains an invalid criterion of causation.