A retrospective cohort study, also called a historic cohort study, is a longitudinal cohort study used in medical and psychological research. A cohort of individuals that share a common exposure factor is compared with another group of equivalent individuals not exposed to that factor, to determine the factor's influence on the incidence of a condition such as disease or death. Retrospective cohort studies have existed for approximately as long as prospective cohort studies.^[1]

The retrospective cohort study compares groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and ones who do not smoke) in terms of a particular outcome (such as lung cancer).^[2] Data on the relevant events for each individual (the form and time of exposure to a factor, the latent period, and the time of any subsequent occurrence of the outcome) are collected from existing records and can immediately be analyzed^[3] to determine the relative risk of the cohort compared to the control group.^[4]

This is fundamentally the same methodology as for a prospective cohort study, except that the retrospective study is performed post-hoc, looking back.^[3] The prospective study looks forward, enrolling patients unaffected by the outcome and observing them to see whether the outcome has occurred.^[3] However, both kinds of cohort studies share the same starting point (considering data from before the occurrence of the outcome). The first objective is still to establish two groups - exposed versus non-exposed - which are then assessed retrospectively to establish the most likely temporal sequence of events leading to the current disease state in both the exposed and unexposed groups.^{[citation needed]}

Retrospective cohort studies require particular caution because errors due to confounding and bias are more common than in prospective studies.^[4]

Retrospective cohort studies exhibit the benefits of cohort studies and have distinct advantages relative to prospective ones:

• They are conducted on a smaller scale.^[5]
• They typically require less time to complete.^[5]
• They are generally less expensive, because resources are mainly devoted to collecting data.^[6]
• They are better for analyzing multiple outcomes.^[6]
• In a medical context, they can potentially address rare diseases, which would necessitate extremely large cohorts in prospective studies.^[5]

Retrospective studies are especially helpful in addressing diseases of low incidence, since affected people have already been identified so .^[7] The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit.^[6] Additionally, it has essentially all the benefits of a cohort study.

Retrospective studies have disadvantages vis-a-vis prospective studies:

• Some key statistics cannot be measured, and significant biases may affect the selection of controls.^[5]
• Researchers cannot control exposure or outcome assessment, and instead must rely on others for accurate recordkeeping.^[6] When relying on individual recall of former exposure to risk variables, recall may be inaccurate and subject to biases.^[5] It can be very difficult to make accurate comparisons between the exposed and the non-exposed.^[6]
• The retrospective aspect may introduce selection bias and mis-classification or information bias.^[8] With retrospective studies, the temporal relationship is frequently difficult to assess.^[6]
• Retrospective studies may need very large sample sizes for rare outcomes.^[6]

While retrospective cohort studies try to compare the risk of developing a disease to some already known exposure factors, a case-control study will try to determine the possible exposure factors after a known disease incidence. Both the relative risk and odds ratio are relevant in retrospective cohort studies, but only the odds ratio can be used in case-control studies. Although most case-control studies are retrospective, they can also be prospective when the researcher still enrolls participants based on the occurrence of a disease as new cases occur.^{[citation needed]}

• Case-control study