Sunday, January 20, 2008

The Ghost Map

If you're anything like me, then when you learned about Jon Snow and the Broad Street Pump, you learned something like this:
  1. There was a cholera epidemic in an area of Soho in London in 1854.
  2. Jon Snow was a doctor there, who drew a map of where the deaths were, and realized that water coming from the Broad Street pump was the source.
  3. Upon realizing that, he removed the handle from the pump, to stop people using it.
  4. Snow showed that cholera was an infectious disease.
  5. He thereby also founded the discipline of epidemiology.
Steven Johnson has written a book, called The Ghost Map, named after the map that was drawn by Snow, all about the incident. (Johnson also wrote Everything Bad is Good for You).

The first thing you learn from the book is that the popular account of the Broad Street pump incident is wrong in many, many ways. The events happened in almost the reverse order. Epidemiology existed before the incident (not long before, and Snow was a founder). Some people thought that cholera was infectious before this incident. Most didn't. Most didn't think it was infectious afterwards either. Snow removed the pump handle - but before drew the map. He didn't even draw the best map - someone else drew a better improved version later on.

The book also goes into detail about what was known about the minor details of people's lives, where they worked (this was important, because this affected what they drank) where their relatives lived, but sometimes details are lost - no one knows the name of the first victim. It examines the epidemic on a broader, political scale - why had things got to the state where it was possible for a cholera epidemic to occur. It examines how the situation arose that cholera could spread - the city got too big, the carts that had to carry the, errrmmm... , waste had too far to go, and removing the 'night soil' became too expensive; people filled their cellars with shit instead. It looks at what happened as a result - sewage systems removed sewage from people's houses, and dumped it in the river.

So why am I writing about it here? It shows how three things that are required for statistical analysis to have an effect come together. First, there was a theory - the theory could be tested by the collection of data. Snow (and others) spend a long time collecting that data, finding who had died, where they had lived, what they did, but the theory was needed to guide the data collection. Second, the data had to be presented clearly - in this case on the map, which was a form of bar chart, with a bar on each house. (Nowadays we'd call that a Geographical Information System (GIS).) Third, the consumers of research had to be persuaded - in this case, the government departments who needed to build sewers to improve public health.

Finally, the book is extraordinarily well written. It manages to keep the text flowing without becoming turgid, and without deviating from the facts, here's a brief example: "Word of the outbreak had traveled through the wider city and beyond. The chemist's son who had enjoyed his pudding days before on Wardour Street died on that Sunday at his home in Willesden."

There's more information in the John Snow entry on Wikipedia.

posted by J @ 4:44 AM 0 Comments