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Observational epidemiologic studies are a type of nonexperimental research in which exposure is not controlled by the investigator. Observational studies are by far the most common form of clinical research because of their relatively low complexity, cost, and ethical constraints compared to randomized trials or other forms of clinical experimentation. Bias, confounding, and issues with validity are more common in observational studies. Observational studies can be retrospective or, in some cases, prospective. Common forms of observational studies in clinical research include cross-sectional studies, ecologic studies, case-control studies, and cohort studies.