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The measure of risk in a case control, the odds ratio, as a valid estimator of the relative risk, is based on the assumption that:
  • the disease is rare
  • the cases are incident cases drawn from a known and defined population
  • the controls are drawn from the same defined population and would have been in the case group if they had developed the disease
  • controls are selected in an unbiased way, e.g. independently of exposure status.