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Cohort Study A study that makes observations about the association between a particular exposure or risk factor (e.g. tobacco use) and the subsequent development of a disease or condition (e.g. lung cancer). In these studies, subjects do not presently have the condition of interest (lung cancer) and are followed over time to see at what frequency they develop the disease/condition as compared with a control group that is not exposed to the risk factor (tobacco use) under investigation.