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Although many people may associate the name “John Snow” with the character from
the
Game of Thrones
fantasy novels, students of epidemiology may be more likely than
most to associate this name with a 19th century physician who unraveled the mystery
behind a deadly 1854 cholera outbreak near Broad Street in London.
The cholera outbreaks in the city during the 1850s claimed the lives of tens of
thousands of people. At the time, the prevailing medical consensus was that the vehicle
for cholera transmission was a miasma, or “bad air,” perhaps originating from pollu-
tion. Complicating matters were ideas that people from lower economic classes were
more naturally susceptible to disease (a result of a supposed “moral depravity” of the
poor
2
) and that these poor residents tended to create more pollution due to overcrowd-
ing and lack of resources.
John Snow's investigations into the cholera outbreaks eventually led him to believe
that the miasma theory for cholera transmission was faulty. Snow used a variety of
techniques to study the tragedy, including working with Reverend Henry Whitehead,
a local expert on the behavior of people who lived in the London outbreak zone. One
of Snow's tools to convince others that the outbreak was due to an infected water
supply from a particular pump was to create a map plotting the deaths. Without hav-
ing any knowledge of the germ theory of disease transmission (which was not well
regarded at the time), Snow came to the conclusion that the epicenter of the out-
break was a pump central to the deadly cases he plotted. This was only part of the
story; Snow then needed to convince local authorities as well as fellow scientists of his
theory—and he did so with the help of a type of visualization now known as a
dot-
distribution map
(see Figure 7.1). His resulting Broad Street pump cholera map is an oft-
cited milestone in the development of using cartography for understanding and, more
importantly, communicating the impact of medical outbreaks.
3
The well-known information visualization pioneer and author Edward Tufte makes
the claim that an 1869 work by Charles Joseph Minard “may well be the best statistical
graphic ever drawn,”
4
and it's difficult to disagree with this assessment. Minard's chart,
titled
Carte figurative des pertes successives en hommes de l'Armée Française dans la campagne
de Russie 1812-1813
5
,
tells the statistical story of Napoleon's disastrous foray into Rus-
sia during the campaign of 1812 (see Figure 7.2). Not only is the work aesthetically
appealing, but it packs an amazing amount of information into a single chart. Minard's
work is an example of a
flow map
, which combines aspects of area charts and maps.
The representation of the numbers of troops using dynamically sized bars begins with
a thick, light-colored bar depicting the massive and confident army of nearly half a
2. Johnson, Steven.
The Ghost Map
. New York: Riverhead, 2006.
4. Tufte, Edward R.
The Visual Display of Quantitative Information.
New York: Graphics Press,
1983, p. 40.
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