The grand experiment was described by John Snow in Part 3 of his book. He wrote of the times:
"London was without cholera from the latter part of 1849 to August 1853. During this interval an important change had taken place in the water supply of several of the south districts of London. The Lambeth Company removed their water works in 1852 from opposite Hungerford Market to Thames Ditton [actually Seething Wells]; thus obtaining a supply of water quite free from the sewage of London."
- Snow, 1855, p. 68.
He went on to state:
The districts supplied by the Lambeth Company are, however, also supplied, to a certain extent, by the Southwark and Vauxhall Company, the pipes of both companies going down every street, in the places where the supply is mixed..."
- Snow, 1855, p. 68.
Thus two water companies supplied the same neighborhoods with water, with one company having moved to a fresh water site while the other remained in a polluted site. During the 1849 epidemic, the water of both companies was drawn from the same contaminated region of River Thames, and the death rates among their consumers were similar. When cholera reappeared in 1853-54, however, the exposure to polluted water had changed, establishing the basis for a natural experiment.
Of the unusual natural situation, Snow wrote:
"The experiment, too, was on the grandest scale. No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentlefolks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied with water containing the sewage of London, and, amongst it, whatever might have come from the cholera patients, the other group having water quite free from such impurity."
- Snow, 1855, p. 75.
This was not a true experiment since Snow did not randomly allocated people into two groups, one exposed to contaminated water and the other not. Clearly, this would have been unethical and certainly illegal. He did, however, recognize that allocation and near-randomization had taken place in a natural setting, and had taken advantage of this historical occurrence to test his hypothesis. He used the classic experimental design shown below to analyze his data and derive important conclusions for the eventual control of cholera.
Source: Snow J. On the Mode of Communication of Cholera, 1855.