A group of persons with a common characteristic or set of characteristics. Typically, the group is followed for a specified period of time to determine the incidence of a disorder or complications of an established disorder (prognosis).
This is a investigation in which a cohort of individuals who do not have evidence of an outcome of interest but who are exposed to the putative cause are compared with a concurrent cohort who are also free of the outcome but not exposed to the putative cause. Both cohorts are then followed forward in time to compare the incidence of the outcome of interest. When used to study the effectiveness of an intervention, it is an investigation in which a cohort of individuals who receive the intervention are compared with a concurrent cohort who do not receive the intervention, wherein both cohorts are followed forward to compare the incidence of the outcome of interest. Cohort studies can be conducted retrospectively in the sense that someone other than the investigator has followed patients, and the investigator obtains the data base and then examines the association between exposure and outcome.
An observational study can be used to describe many designs that are not randomized trials (e.g., cohort or case-control studies that have a goal of establishing causation, studies of prognosis, studies of diagnostic tests, and qualitative studies). The term is most often used in the context of cohort and case-control studies in which patient or caregiver preference, or happenstance, determines whether a person is exposed to an intervention or putative harmful agent or behavior (in contrast to the exposure being under the control of the investigator as in a randomized trial).