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:
“In youth the human body drew me and was the object of my secret and natural dreams. But body after body has taken away from me that sensual phosphorescence which my youth delighted in. Within me is no disturbing interplay now, but only the steady currents of adaptation and of sympathy.”
—Haniel Long (1888–1956)
“Mathematics alone make us feel the limits of our intelligence. For we can always suppose in the case of an experiment that it is inexplicable because we don’t happen to have all the data. In mathematics we have all the data ... and yet we don’t understand. We always come back to the contemplation of our human wretchedness. What force is in relation to our will, the impenetrable opacity of mathematics is in relation to our intelligence.”
—Simone Weil (1909–1943)