1 Preventive Medicine Department, Federal University of Bahia, Av. Reitor Miguel Calmon, s/n Vale do Canela, Salvador, Bahia, Brasil CEP 40.420-060
2 Department of Work Environment, University of Massachusetts, Lowell, MA 01854
Recently, during an epidemiology lecture for undergraduates, one student noticed that theres a mistake in John Snows calculation of the death rate of cholera for the "rest of London" in the 1854 epidemic described in On the Mode of Communication of Cholera (1). The correct rate, derived by dividing 1,422 deaths by 256,423 houses, should be 55 instead of 59 deaths in each of 10,000 houses, as presented in Snows table IX, reprinted below as table 1.
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This minor mistake has no bearing on the overall conclusions nor the importance of Snows work. What is, perhaps, worthy of comment is the fact that the mistake has been very often repeated in epidemiology textbooks in which the story has been told (210). Snows fieldwork has become the paragon of scientific inquiry, presented as the classic epidemiologic case study leading to preventive action. On the Mode of Communication of Cholera is considered a turning point in the epidemiologic research paradigm. Snows work is used to demonstrate the importance of high-quality primary data as the basic material for the investigative process that epitomizes epidemiologic reasoning.
All over the world, introductory courses of epidemiology start with the classic exercise, "Snow on cholera" (10), drawing upon tables and maps of the London cholera epidemic of 1854. As Vandenbroucke et al. (11) reported, the didactic use of this case study can be traced back at least as far as Wade Hampton Frost in his pioneering epidemiology course at Johns Hopkins University. A casual survey of epidemiology textbooks revealed that at least nine have reproduced Snows table IX exactly as it was presented in his book, without noting or correcting the error (2, p. 25; 3, p. 17; 4, p. 46; 5, p. 9; 6, p. 7; 7, p. 21; 8, p. 7; 9, p. 9; 10, p. 435). As a learning tool, we encourage our fellow teachers of epidemiology to continue to use Snows important book, but even heroes make mistakes, and errors in long division should not be passed along to our students.
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