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Experimental studies are the definitive studies in Epidemiology
as in other medical studies. They are not always possible,
either because unethical as in studies inflicting pain or
removing vital organs (as in the Nazi studies in World War II),
or because impractical (asking people to smoke or work in a coal
mine, etc.).
Where possible, they can either involve individuals or
communities. The latter are more uncommon. A prime example
was the study done in New York State where one community had
its water supply fluoridated while a similar community did not.
After baseline studies of tooth decay in children, a follow-up
study some years later showed a reduction in tooth decay in the
experimental community, but no change in the other.
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