These two tables were constructed by Dr. Snow in pursuit of his hypothesis that cholera was transmitted by contaminated water through an unknown mechanism. Between 1849 and 1854, the Lambeth Company changed its source to an area of the Thames where the water was “quite free from the sewage of London”; the dramatic difference in cholera rates were the result.

This is an example of testing an hypotheses using a natural experiment. In the first table, the row “both companies” refers to neighborhoods in which the supply was mixed, and door-to-door inquiry was needed to determine exactly which company was the supplier of the water and ultimately the cholera-causing organisms.