© Lorraine Kay
(4) Doll and Hill used epidemiology to prove that deaths from smoking increase with the amount consumed.
© Lorraine Kay
(4) Epidemiological studies proved that fluoride helps reduce tooth decay.
Hypotheses can also be tested by experimental trials in which different groups of people are exposed to differing amounts of the test substance. A classic example is the use of fluoride to reduce dental caries.10
A practicing dentist noticed that children with mottled teeth, caused by a high concentration of fluoride in the water supply, seemed to have less tooth decay than usual. This prompted the Public Health Service to initiate epidemiological surveys of children from cities where the fluoride concentration varied considerably. The results indicated that dental caries decreased with increasing content of fluoride in the water. But final proof was only obtained through an epidemiological experiment in which fluoride was added to the water supply of one community and the subsequent dental experience of school children compared with another town with little or no fluoride in the water.
An early epidemiological experiment is said to have ended the fashionable practice of bleeding as a medical treatment.11 In 1835, Pierre Louis studied the outcome of pneumonia in patients hospitalized in Paris and discovered that bleeding increased the death rate. Until then, millions of leeches were imported into Paris every year.
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