In 1854, a public health crisis was occurring in Soho, London, with a cholera outbreak in the neighborhood around Broad Street. By talking to residents, English physician John Snow suspected that the contamination source was the now infamous Broad Street water pump, so he had the pump handle removed. Later, he quantified the outbreak using a map that visualized the source of the contamination, which confirmed that the Broad Street water pump was the problem. By solving that public health mystery, Snow is now celebrated as one of the founders of modern day epidemiology. Given the same data points today, how would the exploration of the data look using a 21st century lens?
John Snow's Original Map, drawn and lithographed by Charles Cheffins.
Source:https://en.wikipedia.org/wiki/John_Snow
Source: https://en.wikipedia.org/wiki/John_Snow
The data points from Snow’s original map include the location and number of cholera cases at each residence, as well as the location of the nearby water pumps. Broad Street is now named Broadwick Street in present-day London. Plotting the longitude and latitude of the points in JMP’s Graph Builder, we get a visualization of the residences reporting cases, as well as the water pump locations.
In different iterations by Snow of the original map, he denoted the density of the cholera cases, both as bars and as dots on the map. At first glance, by having the size of the marker denote the number of cases (as shown in the map below), we start to see locations of interest, but it is difficult to determine relative size of the circle markers to denote density of the cases.
The data set we are working with contains the specific addresses of the affected households, making it easy to extract the street name. Upon closer examination, we see that there are four streets that have greater than 5% of the total cases.
Using the dynamic linking in JMP by selecting the four bars in the chart, the selected streets are highlighted back in our map. Notice the location of the water pump on Broadwick Street, formerly Broad Street. A clearer picture emerges.
Today, we have additional tools that can show the density of cases more effectively than points alone could. To show the patterns in density, nonparametric density can be added to the map using JMP’s Fit Y by X platform. The quantile contour lines are added at 5% intervals. The density contours clearly show the hot spot of the cholera cases, with the Broad Street water pump right in the center.
Modern visualization techniques that reveal density allow this 19th century data to be explored in ways that were not available to Snow. However, the overarching goal for graphing this data set is the same today as it was in Snow’s day: concisely communicate that the Broad Street water pump marked the spot of the outbreak’s epicenter.
References
Images: https://en.wikipedia.org/wiki/John_Snow
Data Source: http://blog.rtwilson.com/updated-snow-gis-data/
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