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Crosier: John Snow

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Crosier. 2003. John Snow: The London Cholera Epidemic of 1854. from course CD, originally from a web site..

Problems:

  • nonrandom selection
  • mortality threat: all the sick people died
  • regression to the mean
  • thus, can't conclude that vapors don't cause cholera.

If nature gives us a natural experiment, how do we know whether it is roughly random? (1) the subjects don't know which group they were assigned to, (2) we don't either. The cholera experiment looks close enough to this test.