Snow exploited this natural event and carried out house to house survey of about 300,000 users of both companies and confirmed his hypothesis that cholera deaths were due to the contaminated water supplied by Southwark company by comparing the death rates of both companies, by observing the reduction in mortality after changing the source by Lambeth company and by excluding other probable confounding factors. He prepared and used a spot map showing deaths and cases and found clustering of cases in Broad street. He removed the head of the tap and cases declined in that area. His experiment in his own words.