Snow’s “Grand Experiment”
-------------------------

Encouraged by what he had learned in Soho, Snow completed a more thorough
analysis. For some time, he had been gathering data on cholera
deaths in an area of London that was served by two water companies. The Lambeth
water company drew its water upriver from where sewage was discharged into the
River 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 was
contaminated.

The map below shows the areas served by the two companies. Snow honed in on the region where the two service areas overlap.
![Snow’s Other Map](../../../images/snow_map2.jpg)

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

The only difference was in the water supply, “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
impurity.”

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

| Supply Area    | Number of houses | cholera deaths | deaths per 10,000 houses |
|----------------|------------------|----------------|--------------------------|
| S&V            | 40,046           | 1,263          | 315                      |
| Lambeth        | 26,107           | 98             | 37                       |
| Rest of London | 256,423          | 1,422          | 59                       |


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