A retrospective cohort study, also called a historic cohort study, is a

longitudinal Longitudinal is a geometric term of location which may refer to: * Longitude ** Line of longitude, also called a meridian * Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, ...

cohort study A cohort study is a particular form of longitudinal study that samples a cohort (a group of people who share a defining characteristic, typically those who experienced a common event in a selected period, such as birth or graduation), performing ...

used in

medical Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care practi ...

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.

Design

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 Outcome may refer to: * Outcome (probability), the result of an experiment in probability theory * Outcome (game theory), the result of players' decisions in game theory * ''The Outcome'', a 2005 Spanish film * An outcome measure (or endpoint) ...

(such as

lung cancer Lung cancer, also known as lung carcinoma (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissues of the lung. Lung carcinomas derive from transformed, malign ...

). 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 to determine the

relative risk The relative risk (RR) or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. Together with risk difference and odds ratio, relative risk measures the association b ...

of the cohort compared to the control group. This is fundamentally the same methodology as for a prospective cohort study, except that the retrospective study is performed post-hoc, looking back. The prospective study looks forward, enrolling patients unaffected by the outcome and observing them to see whether the outcome has occurred. 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. Retrospective cohort studies require particular caution because errors due to confounding and bias are more common than in prospective studies.

Advantages

Retrospective cohort studies exhibit the benefits of

cohort studies A cohort study is a particular form of longitudinal study that samples a cohort (a group of people who share a defining characteristic, typically those who experienced a common event in a selected period, such as birth or graduation), performing ...

and have distinct advantages relative to prospective ones: *They are conducted on a smaller scale. *They typically require less time to complete. *They are generally less expensive, because resources are mainly devoted to collecting data. *They are better for analyzing multiple outcomes. *In a medical context, they can potentially address

rare disease A rare disease is any disease that affects a small percentage of the population. In some parts of the world, an orphan disease is a rare disease whose rarity means there is a lack of a market large enough to gain support and resources for discov ...

s, which would necessitate extremely large cohorts in prospective studies. Retrospective studies are especially helpful in addressing diseases of low incidence, since affected people have already been identified so . The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit. Additionally, it has essentially all the benefits of a

Disadvantages

Retrospective studies have disadvantages vis-a-vis prospective studies: *Some key statistics cannot be measured, and significant biases may affect the selection of controls. *Researchers cannot control exposure or outcome assessment, and instead must rely on others for accurate recordkeeping. When relying on individual recall of former exposure to risk variables, recall may be inaccurate and subject to biases. It can be very difficult to make accurate comparisons between the exposed and the non-exposed. *The retrospective aspect may introduce

selection bias Selection bias is the bias introduced by the selection of individuals, groups, or data for analysis in such a way that proper randomization is not achieved, thereby failing to ensure that the sample obtained is representative of the population int ...

and mis-classification or information bias.https://onlinecourses.science.psu.edu/stat507/node/54 With retrospective studies, the temporal relationship is frequently difficult to assess. *Retrospective studies may need very large sample sizes for rare outcomes.

Comparison with case-control studies

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.

References

Citations

Sources

* {{Medical research studies Cohort study methods Clinical research Nursing research