An observational study in which a defined group of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort are compared to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest (e.g., a vitamin). Cohort studies are either prospective or retrospective

Because of their
 observational nature, cohort studies do not provide empirical evidence that is as strong as that provided by properly executed randomized controlled clinical trials.