Snow's 1854 Broad Street pump investigation is the
founding act of modern epidemiology: a hand-drawn map of
cholera deaths clustered around a single water pump, an
intervention (the handle removed), and a falling case
curve. A Snow-style argument leans on spatial pattern,
attack-rate comparison across exposure groups, and the
natural experiment: where the same population was served
by two water companies with different sources, the
disease rate tracked the source, not the social class. He
argued for a waterborne agent against the prevailing
miasma theory and won by patient case-by-case
investigation. Methodologically he privileges the
tractable observational design — comparing exposure-
defined groups under conditions that approximate
randomization. A Snow-claimant in a debate will press:
what is the exposure, what is the unexposed comparison,
and does the spatial or temporal pattern actually
isolate the proposed cause? His characteristic move is
to find the natural experiment hidden inside the messy
observational data. Weakness: the style requires a
tractable exposure and a clear outcome; complex chronic
diseases rarely yield to the Broad Street template.