Masters of ’Metrics: John Snow

British physician John Snow was one of the fathers of modern epidemiology, the study of how illness moves through a population. Studying an outbreak of cholera in London in 1849, Snow challenged the conventional wisdom that the disease is caused by bad air. He thought cholera might be caused by bad water instead, an idea he first laid out in his 1849 essay On the Mode of Communication of Cholera.

A further cholera outbreak in 1853 and 1854 claimed many lives in the London neighborhood of Soho. Snow attributed the Soho epidemic to water from a pump on Broad Street. Not afraid to give a natural experiment a helping hand, he convinced the local parish council to remove the handle of the Broad Street pump. Cholera deaths in Soho subsided soon after, though Snow noted that death rates in his Broad Street treatment zone were already declining, and that this made the data from his natural experiment hard to interpret. DD was as fickle at birth as it is today.

Snow was a meticulous data grubber, setting a standard we still aspire to meet. In an 1855 revision of his essay, Snow reported death rates by district and water source for various parts of London. He noted that many of the high-death-rate districts in South London were supplied by one of two companies, the Southwark and Vauxhall Company or the Lambeth Company. In 1849, both companies drew water from the contaminated Thames in central London. Starting in 1852, however, the Lambeth Company drew from the river at Thames Ditton, an uncontaminated water source upstream. Snow showed that between 1849 and 1854 deaths from cholera fell in the area supplied by the Lambeth Company but rose in that supplied by the Southwark and Vauxhall Company. Our Figure 5.7 reproduces Table 12 from Snow’s 1855 essay.— This table contains the ingredients for Snow’s two-period DD analysis of death rates by water source.

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