Epidemiologists typically identify two major types of study designs: experimental and observational. The principal distinction is whether the treatment or intervention is under the control of the investigator. In experimental studies, the investigator introduces a factor or intervenes in the environment of the study subjects and observes the impact of the intervention on the study subjects. In observational studies, the investigators describe the events that naturally occur without their direct intervention. The three principal observational designs used by epidemiologists, cross-sectional, case-control (or group comparison), and cohort studies, are described below.
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