Concurrent design: OR estimates Relative Rates Controls representative proportion of population at risk when the case appears (concurrent selection) Represent person-time at risk in exposed and unexposed A control can be a case later A person can be a control for several cases Matched analysis because of time matching Example: Prolonged OB of hepatitis C in a dialysis unit selecting 3 controls per case among those at risk of infection at the same time as the case occurs
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