Cohort studies are observational analytical studies and are placed just below randomized controlled trials in the hierarchy of scientific evidence. They are also called longitudinal studies and can be prospective, retrospective or ambidirectional. In cohort studies, groups (cohorts) that are identical in every aspect other than the exposure are followed up for the outcome. Critical components of a cohort study are the definition of the objective/s, choice of the study population and comparison group, measurement of exposures and outcomes, follow-up, analysis and interpretation. Assembling cohorts, accurate measurement of exposure and outcome, and addressing dropout are the main design imperatives in cohort studies. The advantages and disadvantages of cohort studies and an overview of analysis are also presented.
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