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Cohort study

1. Who is exposed?

   • Define exposure levels
   • Measure it in quantifiable degree (no of cigarettes / day).
   • Prospective: look from exposure to outcome.

2. Who is an appropriate control (unexposed)

   • Should be similar to exposed except for the lack of exposure.
   • Internal source for unexposed is the best (same time and place I.e. hospital).
   • External source comes next (workers in other factory).

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