Snow tested his hypothesis by making comparisons between what?

• A. Groups of people
• B. Laboratory animals
• C. Water samples from different sources
• D. Time-series data from a single location

Answer: (A)

The main idea here is testing a hypothesis by comparing outcomes across groups that have different exposures. John Snow did just that with cholera: he looked at who became sick in areas served by different water sources and who did not. By mapping cases to the water supply and comparing cholera incidence among groups of people who drank from the Broad Street pump versus those who used other pumps, he could see whether exposure to a particular water source was associated with higher disease risk. This group-to-group comparison is what makes his approach a classic observational test of an exposure–disease link. It’s not about testing water samples in the lab, not about time-series data from a single location, and not about animal experiments.