Last reviewed:
John Snow
1813–1858
English doctor seen as a founder of modern epidemiology.
What they got right
He mapped cholera deaths around the Broad Street pump (1854) and compared customers of two water companies, a 'natural experiment' pointing to dirty water, not bad air.
Skill it shows
Evidence and sources
Practise this skillAlso: Numbers and chance
The result of the inquiry then was, that there had been no particular outbreak or increase of cholera, in this part of London, except among the persons who were in the habit of drinking the water of the above-mentioned pump-well.
Worth knowing
The outbreak was already falling when the pump handle was removed, and his water theory was only widely accepted after his death.
Try it yourself
Something got better after a change was made. What else would you need to know before saying the change caused it?