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:
“We do not want them to have less.
But it is only natural that we should think we have not enough.
We drive on, we drive on.
When we speak to each other our voices are a little gruff.”
—Gwendolyn Brooks (b. 1917)
“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)