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Retrospective cohort studies are often conducted before prospective cohort studies to validate evidence gathered using weaker study designs (e.g., descriptive studies) concerning the association between the exposure and the outcome. If the risk of disease was higher (or lower) in the exposed group, we conclude that the exposure is associated with increased (or decreased) risk of the disease. If the risk of disease in the two groups was not different, we conclude that there is no association between the exposure and the disease. The interpretation of the findings from retrospective cohort studies is similar to that of findings from prospective cohort studies. Therefore, investigators usually create two groups of people who are known to have been either exposed or not exposed to the factor of interest during a specific time frame, and then they compare the two groups with regard to their disease status. However, in retrospective studies, the research is initiated after both the exposure and the outcome (e.g., disease) have already occurred. Like prospective cohort studies retrospective cohort studies classify study participants on the basis of whether or not they were exposed to the factor under investigation. Retrospective cohort studies are a type of observational research in which the investigator looks back in time at archived or self-report data to examine whether the risk of disease was different between exposed and non-exposed patients. Retrospective cohort study design: An observational epidemiologic design in which risk of disease is retrospectively compared between an exposed and a non-exposed group