True incidence cannot be calculated in case control

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challenge in case control studies is to identify the appropriate control group. True incidence cannot be calculated in case control studies, since the subjects are selected based on disease status. Appropriate measure of association: Odds ratio can be used as a measure of association between exposure and disease in a case control study. In case-control study RR cannot be calculated directly to determine the association between exposure and disease since we don’t know the risk of disease among exposed and un-exposed since we start recruiting cases and controls. 2. Cohort Studies Cohort studies select subjects based on their exposure status. The study subjects should be at risk of the outcome under investigation at the beginning of the cohort study; this means that they should be disease free when the cohort study starts. The cohort is followed through time to assess their later outcome status. In cohort study, underlying population is fixed. [Morabia A] Characteristics: In cohort studies, confounders can be more easily controlled. A cohort study is also efficient for investigating a rare exposure. Cohort study design can be used in situations where Exposure is rare.
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