Cohort Studies

A cohort study is a design in which the investigator selects a group of exposed individuals and a group of unexposed individuals and follows both groups to compare the frequency with which the disease occurs in each group.

In a prospective cohort study, participants are grouped on the basis of past or current exposure and are followed into the future in order to observe the outcomes of interest. When the study commences, the outcomes have not yet developed and the investigator must wait for them to occur.

In a retrospective cohort study, both the exposures and outcomes have already occurred when the study begins. Thus, this type of investigation studies only prior outcomes and not future ones.

An ambidirectional cohort study has both prospective and retrospective components. Researchers who conduct cohort studies typically rely on many sources for information on exposures, outcomes, and other key variables. Data sources include medical and employment records, interviews, direct physical examinations, laboratory tests, biological specimens, and environmental monitoring.

Selection bias and misclassification are among the concerns for cohort studies. Loss to follow-up occurs when researchers lose contact with study participants. Misclassification occurs by inaccurately classifying the exposure status of individuals. Strengths of cohort studies include the ability to determine the time sequence of events and several health outcomes can be assessed

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