NATURAL EXPERIMENT

A 'natural experiment' is a naturally occurring instance of observable phenomena which approach or duplicate a scientific experiment.
An example of a natural experiment occurred in Helena, Montana during the period from June 2002 to December 2002 when a smoking ban was in effect in all public spaces in Helena including bars and restaurants. Helena is geographically isolated and served by only one hospital. It was observed that the rate of heart attacks dropped by 60% while the smoking ban was in effect. Sargent RP, Shepard RM, Glantz SA, 'Reduced incidence of admissions for myocardial infarction associated with public smoking ban: before and after study.', British Medical Journal, volume 328 pp. 977-980, 2004
A further example is the spread of early humans across the Pacific Ocean. The distribution of populations between islands was essentially random, allowing researchers to treat different groups as independent societies drawn from a common pool. Hypotheses can then be tested in different contexts without fear that an unobserved factor is the cause of differences between island populations. This procedure would be inappropriate in studying European colonies, which were typically planned by the metropolitan government and staffed by carefully-chosen populations.

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