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Natural experiment
A natural experiment is an empirical study in which individuals (or clusters of individuals) exposed to the experimental and control conditions are determined by nature or by other factors outside the control of the investigators, yet the process governing the exposures arguably resembles random assignment. 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 exposure involving a well defined subpopulation (and the absence of exposure in a similar subpopulation) such that changes in outcomes may be plausibly attributed to the exposure. In this sense the difference between a natural experiment and a non-experimental observational study is that the former includes a comparison of conditions that pave the way for causal inference, while the latter does not. Natural experiments are employed as study designs when controlled experimentation is extremely difficult to implement or unethical, such as in several research areas addressed by epidemiology (e.g., evaluating the health impact of varying degrees of exposure to ionizing radiation in people living near Hiroshima at the time of the atomic blast) and economics (e.g., estimating the economic return on amount of schooling in US adults).〔〔
==History==
One of the most famous early natural experiments was the 1854 Broad Street cholera outbreak in London, England.
On 31 August 1854, a major outbreak of cholera struck Soho. Over the next three days 127 people near Broad Street died. By the end of the outbreak 616 people died. The physician John Snow identified the source of the outbreak as the nearest public water pump, using a map of deaths and illness that revealed a cluster of cases around the pump.
In this example, Snow discovered a strong association between the use of the water and deaths and illnesses due to cholera. Snow found that the Southwark and Vauxhall Waterworks Company, which supplied water to districts with high attack rates, obtained the water from the Thames downstream from where raw sewage was discharged into the river. By contrast, districts that were supplied water by the Lambeth Waterworks Company, which obtained water upstream from the points of sewage discharge, had low attack rates. Given the near-haphazard patchwork development of the water supply in mid-Nineteenth Century London, Snow viewed the developments as "an experiment...on the grandest scale."〔Snow, J. (1855). ''On the Mode of Communication of Cholera'' (2nd ed.). London: Churchill. Excerpted in MacMahon, B. & Pugh, T.F. (1970). ''Epidemiology''. Boston: Little Brown.〕 Of course, the exposure to the polluted water was not under the control of any scientist. Therefore, this exposure has been recognized as being a natural experiment.〔The 1854 cholera outbreak is the example of a natural experiment discussed often by David A. Freedman, e.g. in ''Statistical Models: Theory and Practice'' (Cambridge University Press) (), chapter 1.3 (pages 6-9).〕〔MacMahon, B., & Pugh, T.F. (1970). ''Epidemiology: Principles and Methods.'' Boston: Little, Brown.〕〔Snow's studies of the pattern of the disease were convincing enough to persuade the local council to disable the well pump by ''removing its handle''. After the handle of the well-pump was replaced, the incidence of new cases dropped. In stopping the use of water from the well-pump, the authorities did an uncontrolled experiment (without a control group) and without randomization.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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