Editing Natural experiment (section)

==History==
[[File:Snow-cholera-map-1.jpg|thumb|right|220px|[[John Snow (physician)|John Snow]]'s map showing the clustering of [[cholera]] cases in [[Soho]] during the London epidemic of 1854]]
One of the best-known 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 (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 (epidemiology)|cluster]] of cases around the pump.<ref>MacMahon B, Pugh TF. ''Epidemiologic Methods'' (Little, Brown; 1960); reissued as ''Epidemiology: Principles and Methods'' (Little, Brown; 1970) ({{ISBN|0316542598}})</ref>

In this example, Snow discovered a strong association between the use of the water from the pump, 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 [[River Thames|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."<ref>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.</ref> 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.<ref>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) [http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521671057], chapter 1.3 (pp.  6–9).</ref><ref>MacMahon, B., & Pugh, T.F. (1970). ''Epidemiology: Principles and Methods.'' Boston: Little, Brown.</ref><ref>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 conducted, in effect, a second study, an uncontrolled intervention study, a study with an intervention group but no control group.</ref>