Re-evaluating John Snow's 1856 south London study.

John Snow, the London doctor who studied cholera in the 1840s and 1850s, argued in Snow (1856) that water exerted an "overwhelming influence" on mortality in a region of south London during the 1854 outbreak. In a paper re-assessing Snow's analysis, Koch and Denike (2006) claim that "Snow made not merely minor arithmetic errors but more importantly critical, conceptual mistakes that adversely affected his results." The claim of errors and mistakes is incorrect and due to a misreading or misunderstanding of Snow's data and analysis. Koch and Denike apply an inappropriate statistical test to Snow's original data (and do so incorrectly). More importantly, due to the misreading of the historical record they alter the underlying primary-source data, rendering their results invalid. Analysis of the data following Snow's approach but with modern statistical tools strongly supports Snow's claim for the primacy of water in accounting for variation in cholera mortality.