Scientific Method John Snow, William

¶ … Scientific Method

John Snow, William Farr: Cholera and the scientific method

John Snow's thesis

John Snow, a Victorian epidemiologist and physician to Queen Victoria, constructed his (accurate) hypothesis that cholera epidemics were caused by poor sanitation and 'morbid material' in the sufferers' guts based upon his analysis of the disease's physical manifestations and progression. The disease was marked by pain, vomiting, diarrhea, and dehydration and responded to conventional treatments for gut-related disorders. All of this suggested that the sources of cholera entered the body through swallowing. Additionally, examining the water quality of places with known outbreaks of cholera caused Snow to conclude that cholera mortality in these areas was fourteen times higher among with uncontaminated or less contaminated water (Eyler 2001: 225).

Snow's deployment of the scientific method of testing a hypothesis involved a series of experiments, using different types of evidence to support his conclusion. Snow reconstructed how the water supply of a row of 17 houses could have become contaminated from cesspools and surface water drains. Then Snow "examined a specimen of water from an outbreak center" and noted that it "smelled like privy soil and contained bits of undigested food that had clearly passed through the alimentary canal" (Eyler 2001: 226). Combined with the epidemiological evidence compiled by William Farr, Superintendent of the General Register Office, England's center for vital statistics, Snow confirmed his belief that contaminated water was the source of cholera outbreaks.

Farr's epidemiology

William Farr, Snow's contemporary, also attempted to trace the cholera epidemic over time and geographically throughout the city. Farr investigated the roles of sex, age, seasons, day of the week and soil elevation and how they were correlated with cholera transmission. Snow began with a hypothesis and focused upon specific, real world scenarios that could prove or disprove his thesis. Farr was far broader in his outreach, however, and did not focus upon one source of transmission or data: he simply tried to gather as much evidence as possible. Based upon what he learned, Farr proposed a 'law' or formula "that could predict in mathematical terms human mortality from cholera according to soil elevation" (Morabia 2001: 223). Unfortunately, Farr's so-called law was based upon a fundamentally flawed view of disease transmission: the conventional wisdom at the time was that almost all diseases were caused by airborne molecules. While Farr conceded that the water in the areas of cholera outbreaks was quite dirty, he believed this merely confirmed that cholera was "caused by zymotic (that is, produced by fermentation) factors" off of the water (Morabia 2001: 223). Farr thought that the fumes from sewage-treated water were to blame, not ingesting the cholera bacterium. Farr believed that cholera was caused by an as-yet unidentified zymotic material and while he admitted some of the particles might be ingestible he still thought that most of the toxic materials entered the body through the lungs (Eyler 2001: 228).

Snow vs. Farr

Snow found Farr's data valuable but the two men reached entirely different conclusion based upon Farr's information. In "his Tenth Annual Report of 1847, Farr talks of 'the disease mist, arising from the breath of two millions of people, from open sewers and cesspools, graves and slaughter-houses (that) & #8230; like an angel of death has hovered for centuries over London'…Snow had the correct hypothesis, but Farr had built the most advanced public health surveillance system of that time. This system was instrumental in providing the evidence that convinced the public" that Farr was right when he stated that cholera was spread primarily through the air, not contaminated water (Morabia 2001: 223).

In his essay "Snow and Farr: A scientific duet," Alfredo Morabia characterizes Snow's approach as more brilliant and characterized by leaps in logic, while Farr's techniques, he stated were more methodical. John M. Eyler, in contrast, in his article "The changing assessments of John Snow's and William Farr's cholera studies" characterizes Snow as more dogmatic and tenacious in upholding this thesis, while Farr was more willing to entertain alternative possibilities and dissent. On one hand, Farr's thesis upheld common notions of disease transmission. Snow's theory clashed with conventional models that suggested that 'bad air' was behind the spread of all communicable illnesses (Schoenbach 1999). But on the other hand, Farr was interested in accumulating data first, then forming a conclusion while Snow began with a hypothesis, and then sought to prove the hypothesis right or wrong through a series of highly specific tests.

Snow, in contrast to Farr's epidemiology, was far more innovative and spontaneous in his methods, which also made his conclusions, in the eyes of his colleagues more suspect. As well as doing his own hands-on research, Snow analyzed the "natural experiment created when one water- supply company of London, the Lambeth Company -- but not the Southwark and Vauxhall Company -- moved its water inlet to a less polluted area of the Thames. Snow's hypothesis was that if cholera was related to consumption of water contaminated by human excrements, then mortality rates should be greater among those who drank the contaminated water supplied by the Southwark and Vauxhall Company than among those who drank the cleaner water supplied by the Lambeth Company" (Morabia 2001: 224). Determining the exact purity of the water supply at a given point in time, however, was difficult, and made it difficult for Snow's thesis to eliminate other possibilities of causation (Eyler 2001: 227-288).

Thus from the point-of-view of his colleagues at the time, there was some 'sloppiness' to Snow's published methods and his unequivocal claims, in marked contrast to the cleaner, more balanced portrayal provided by Farr. "Snow did not know the number of people at risk of cholera in his test case…he did not even know the number of households supplied by each company in the districts with the mixed water supply" (Eyler 2001: 227). Because the experiment was based upon real-world data, Snow's experimental analysis was not perfectly controlled. There were distinct demographic differences thought to be relevant at the time between the populations using the different water supplies (Eyler 2001: 227). Snow also did not entertain a multifactoral possibility for the spread of disease. On the surface, Farr seemed more balanced and scientifically impartial: Snow was more passionate and less rigorous in his defense of his belief schema.

In defense of Snow's critics, one of the most common criticisms of many scientific experiments is the unwillingness on the part of the researchers to deal with contradictory data that does not support their original hypothesis. Farr's tentativeness made him seem more fair and scientific. Yet while Snow was criticized for subsuming too much evidence to suit his thesis, his critics could and should be equally criticized for subsuming their view of his research to suit their own ideas about how all disease were airborne.