Snow’s “Grand Experiment”

Encouraged by what he had learned in Soho, Snow completed a more thoroughanalysis of cholera deaths. For some time, he had been gathering data on choleradeaths in an area of London that was served by two water companies. The Lambethwater company drew its water upriver from where sewage was discharged into theRiver Thames. Its water was relatively clean. But the Southwark and Vauxhall(S&V) company drew its water below the sewage discharge, and thus its supply wascontaminated.

The map below shows the areas served by the two companies. Snow honed in on the region where the two service areas overlap.

Snow noticed that there was no systematic difference between the people who weresupplied by S&V and those supplied by Lambeth. “Each company supplies both richand poor, both large houses and small; there is no difference either in thecondition or occupation of the persons receiving the water of the differentCompanies … there is no difference whatever in the houses or the peoplereceiving the supply of the two Water Companies, or in any of the physicalconditions with which they are surrounded …”

The only difference was in the water supply, “one group being supplied withwater containing the sewage of London, and amongst it, whatever might have comefrom the cholera patients, the other group having water quite free fromimpurity.”

Confident that he would be able to arrive at a clear conclusion, Snow summarizedhis data in the table below.

The numbers pointed accusingly at S&V. The death rate from cholera in the S&Vhouses was almost ten times the rate in the houses supplied by Lambeth.

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