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Defining Causation, Specificity and Causality

From: Scientific Studies on Benzene Health Effects

Bradford Hill and Causation

1. Sir Bradford Hill states that his points of view are not logical necessities at all.
2. They are not meant to replace good common sense and unbiased judgment.
3. Indeed, he states that, "Clearly 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 non¹."
4. His points of view must be used and interpreted judiciously. Sir Bradford Hill states:
   • "However, before deducing 'causation' and taking action we shall not invariably have to sit around awaiting the results of the research."
   • "The whole chain may have to be unraveled or a few links may suffice. It will depend on circumstances."

¹ Bradford-Hill A. 1965. The environment and disease: Association or causation? Proc Royal Soc Med 48:295-300.

Specificity

1. "Milk as a carrier of infection and, in that sense, the cause of disease can produce such a disparate galaxy as scarlet fever, diphtheria, tuberculosis, undulant fever, sore throat, dysentery, and typhoid fever."
2. "Before the discovery of the underlying factor, the bacterial origin of disease, harm would have been done by pushing too firmly the need for specificity as a necessary factor before convicting the dairy."

"In short, if specificity exists we may be able to draw conclusions without hesitation; if it is not apparent, we are not thereby necessarily left sitting irresolutely on the fence."

[We can make a common sense decision.]

Finally, Sir Bradford Hill states:

1. "What I do not believe - and this has been suggested - that we can usefully lay down some hard-and-fast rules of evidence that must be obeyed before we can accept cause and effect."
2. "None of my nine viewpoints can bring indisputable evidence for or against the cause-and-effect hypothesis and none can be required as a sine qua non."
3. "What they can do, with greater or less strength, is to help as to make up our minds on the fundamental question - is there any other way of explaining the set of facts before us, is there any other way of explaining the set of effects before us, is there any other answer equally, or more likely than cause and effect?"
4. "All Scientific Work is Incomplete - Whether it be Observational or Experimental."
5. "All Scientific Work is Liable to be Upset or Modified by Advancing Knowledge."
6. "That Does Not Confer Upon Us a Freedom to Ignore the Knowledge We Already Have, or to Postpone The Action That It Appears To Demand In A Given Time."

Bradford-Hill's views on causation are consistent with those of Dr. John Snow, a London physician, who found that virtually all of the 83 people who died of cholera in the Golden Square area of London had drawn water from the popular Broad Street pump.

He recommended that the pump handle be removed. Authorities agreed, thereby preventing further infection from that source.

Snow's views on cholera causation were not shared by a majority of scientists, including the Royal College of Physicians (1853-1854) and the General Board of Health (1854).

The link between polluted water and cholera came 30 years later with Koch's discovery of cholera vibrioin 1884. Meanwhile, many lives were saved.

Causality

1. The source of information for 95% certainty determinations include epidemiological and animal studies and take into consideration target tissues or organs, metabolism, mechanism, pathophysiology of disease as relates to chemical exposures, biological plausibility, more likely than not, which is standard methodology used by scientists in the field.
2. The generally accepted standard of scientific probability of data showing a p value of 0.05 or less, a figure generally accepted as demonstrating a likelihood of 95% that the association shown is not due to chance.

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