John Snow, Asiatic Cholera and the inductive-deductive method - republished
Lecture 14: The importance of the outlier
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Last week, we discussed Farr’s Law of Epidemics and the Broad Street outbreak.
In today’s post, we will discuss the importance of the outlier in epidemiology, and on Thursday, we will learn about how Snow addressed substantial critiques of his work.
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Snow’s hypotheses and work were contrary to the prevailing miasma theory. He was not short of critics, but he managed to turn them to his own advantage.
In September 1854, John Snow was investigating the outbreak of pandemic cholera centred around Broad Street in London.
His colleague, Doctor Frazer of Oakley Square, showed Snow the record of a 59-year-old widow who died after 16 hours of agony.
“Dr Fraser also first called my attention to the following circumstances, which are perhaps the most conclusive of all in proving the connection between the Broad Street pump and the outbreak of cholera. In the "Weekly Return of Births and Deaths" of September 9th, the following death is recorded as occurring in the Hampstead district: "At West End, on 2nd September, the widow of a percussion-cap maker, aged 59 years, diarrhoea two hours, cholera epidemics sixteen hours”.
The death of the widow of Hampstead was difficult to reconcile with Snow’s theory as Hampstead had a different source of water than Soho. However, the record entry contained a clue worth following up on.
Snow decided to investigate further and walked 7 km to the deceased's home.
“I was informed by this lady's son that she had not been in the neighborhood of Broad Street for many months. A cart went from Broad Street to West End every day, and it was the custom to take out a large bottle of the water from the pump in Broad Street, as she preferred it. The water was taken on Thursday, 31st August, and she drank of it in the evening, and also on Friday. She was seized with cholera on the evening of the latter day, and died on Saturday, as the above quotation from the register shows. A niece, who was on a visit to this lady, also drank of the water; she returned to her residence, in a high and healthy part of Islington, was attacked with cholera, and died also. There was no cholera at the time, either at West End or in the neighborhood where the niece died. Besides these two persons, only one servant partook of the water at Hampstead West End, and she did not suffer, or, at least, not severely. There were many persons who drank the water from Broad Street pump about the time of the outbreak, without being attacked with cholera; but this does not diminish the evidence respecting the influence of the water, for reasons that will be fully stated in another part of this work”.
The cholera death of a widow in salubrious Hampstead could not be explained at first sight by Snow’s communication theory. Still, a clue in the notification probably caught Snow’s attention: the late person was the widow of a percussion cap factory owner. If you look at Snow’s map of the Golden Square area (Lectures 11 and 12), you will notice such a factory marked at 37 Broad Street. Snow walked the six miles to Hampstead and, as usual, enquired in person. The unrelated death was, in fact, due to exposure to the Broad Street water. The Hampstead and Islington deaths were linked to the same exposure, which blew a hole in Farr’s pyramid. (see Lecture 4).
CONTEMPORARY THEMES
Dr Fraser and his colleagues from the General Health Board Inquiry were aware of the outliers and the exposure (see Lecture 18). The extraordinary fact is that they dismissed them as their heads were full of miasma. So fixed ideas, credos and solid beliefs are contrary to science and its evolution. The importance of outliers lies in the possibility that the extraordinary may help shed light on the ordinary, and theories must fit the facts, not the other way around.
Readings
Maurizio Bonati. Lombardy, an outlier in the COVID-19 pandemic. First posted on cebm.net on April 14, 2020.
Again, the notable difference with those adhering to a pre-determined theory (or ideology ...) who disregard outliers :'anecdotal', or: 'it's normal to have outliers', or, worse: 'disregard, it'll confuse the issue' while those of the calibre of John Snow become very much interested and want to find out why - even if it means walking 7km to find out.
(Personally, I think that scientists true to their vocation are driven by an inexhaustible sense of curiosity, always asking 'why?', and not resting until they have the answer, be it ever so mundane ... )
what John Snow did was really terrible, wasn't it? He started with some observations;
he should instead have ignored such silly things; he should have started with some very firm belief; (he would of course have KNOWN the answer) and single-mindedly advocated it; endlessly; against all the conflicting evidence of observed facts; he should have constantly advocated his belief; denigrated ruthlessly anyone who dared to suggest otherwise ...
only by these means, can $cience advance .......