Causality in the Time of Cholera : John Snow As a Prototype for Causal Inference

The story of John Snow's 1855 treatise "On the mode of communication of cholera" is a rollicking good tale – full of heroism, death, and statistics. But more fundamentally Snow's work is a sustained effort to convince skeptics, through argument and a wide variety of evidence, of the waterborne theory of cholera articulated in the 1849 essay of the same name. Snow's data and analysis provide a template for how to convincingly demonstrate a causal effect, a template as applicable today as in 1855. I consider two of strands of Snow's evidence – the Broad Street outbreak and the south London “Grand Experiment” – as pedagogical examples of using non-experimental data to support a causal effect. In doing so I discuss extensions to Snow's analysis using modern techniques and tools: most importantly difference-in-differences regression and count (Poisson) regression for error analysis in quasi-randomized control experiments. These provide clear and compelling examples of the modern techniques and tools, while confirming and strengthening Snow's original conclusion on the causal effect of water supply on cholera mortality