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"We then call the one object, cause; the other, effect. We suppose that there is some connexion between them; some power in the one, by which it infallibly produces the other, and operates with the greatest certainty and strongest necessity... experience only teaches us, how one event constantly follows another; without instructing us in the secret connexion, which binds them together, and renders them inseparable."

-David Hume

Enquiries Concerning the Human Understanding and Principles of Morals, 1737

Drs. Terence Davidson & Wendy Smith published a research article in the Archives of Otolaryngology, Head & Neck Surgery in July 2010 (citation) that concludes there is a causal relationship between Zicam nasal spray and loss of smell while yet admitting to being an expert witness in a related litigation [TMD].  The title of the article is:  "The Bradford Hill Criteria and Zinc-Induced Anosmia --- A Causality Analysis".  The authors conclude that "...intranasal zinc gluconate causes hyposmia and ansosmia...[and] increased FDA oversight of homeopathic medications is needed to monitor safety..."  Upon further examination of their methods these conclusions seen unfair and forced.  Their methods and conclusions are directly based on a belief system founded by Scottish polymath David Hume in the mid-Eighteenth century.

The Bradford Hill criteria are a product of Sir Austin Bradford Hill (1897-1991), a self-made English statistician and epidemiologist who sought to codify criteria for determining causal association in epidemiology.  Late in his career in 1965 he echoed the teachings of David Hume to apply Hume's logic and insights to problems of causation in epidemiology.  The basis of Hume's work implies that all causality is a product of human experience---not a priori reasoning.  Hume believed that accumulating experience infers cause and effect because that intimate relationship is unknowable.  Thus, Hill developed an inductive system to establish minimum conditions under which effect can be inferred from a variety of experiences, essentially to develop a case based on the strength of assorted empirical evidence of different sorts ("profiling").  Hume, himself, however backs off his own thinking as he was reluctant to say that the past predicts the future with 100% certainty.

These authors apply this scientific "profiling" to both 25 patients they have seen with zinc-induced anosmia and a variety of literature inputs.  They then overlay these observations against the Hill criteria and pronounce a causal relationship between zinc exposure and anosmia (loss of smell).  This inductive reasoning process, reminiscent of a legal brief, when used to establish causation, is unlike the reductionist and deductive logic used in most experimental research.   The argument against Hume/Hill thinking is that (as is likely in this case) the argument can be seen as circular reasoning, or simply "kitchen logic", or that the conclusion is biased by garbage-in resulting in garbage-out .  The application of the Hill criteria to "prove" causation in clinical pharmacology relies on the subjective weaving of various threads of evidence into a fabric of empirical, but in the end, circumstantial evidence.

Donald K. Riker, Ph.D.
Dr. Riker is a former Associate Director, Personal Healthcare at Procter & Gamble, and the last Vice President of R&D & Chief Scientific Officer at Chattem. He is a member of the American Academy of Allergy, Asthma and Immunology, the Society for Neuroscience, Regulatory Affairs Professionals Society, American Society of Pharmacology and Experimental Therapeutics, and the American Society of Clinical Pharmacology & Therapeutics.

Comments

 

0 # Ray Orzechowski 2010-07-29 15:27

Don,

Your of command of English enlightenment philosophy is impressive!

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