A natural experiment is an empirical study in which the experimental conditions (i.e., which units receive which treatment) are determined by nature or by other factors out of the control of the experimenters and yet the treatment assignment process is arguably exogenous. Thus, natural experiments are observational studies and are not controlled in the traditional sense of a randomized experiment. Natural experiments are most useful when there has been a clearly defined and large change in the treatment (or exposure) to a clearly defined subpopulation (and no change to a comparable subpopulation), so that changes in responses may be plausibly attributed to the change in treatments (or exposure).
Natural experiments are considered for study designs whenever controlled experimentation is difficult, such as in many problems in epidemiology and economics.
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Famous quotes containing the words natural and/or experiment:
“A person who can’t pay gets another person who can’t pay to guarantee that he can pay. Like a person with two wooden legs getting another person with two wooden legs to guarantee that he has got two natural legs. It don’t make either of them able to do a walking-match.”
—Charles Dickens (1812–1870)
“What constitutes a real, live human being is more of a mystery than ever these days, and men—each one of whom is a valuable, unique experiment on the part of nature—are shot down wholesale.”
—Hermann Hesse (1877–1962)