Retrospective cohort study

Historic cohort study

Case–control study versus cohort on a timeline. "OR" stands for "odds ratio" and "RR" stands for "relative risk".

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]

## Design

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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]

## Advantages

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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.

## Disadvantages

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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]

## Comparison with case-control studies

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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]

## See also

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* Case-control study

## References

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### Citations

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1. ^ "What is epidemiology?" (PDF). 11 March 2019. Archived (PDF) from the original on 12 August 2017. Retrieved 29 August 2011.
2. ^ "Definition of historic cohort study - NCI Dictionary of Cancer Terms".
3. ^ a b c Mann, C J (1 January 2003). "Observational research methods. Research design II: cohort, cross sectional, and case-control studies". Emergency Medicine Journal. 20 (1). BMJ Publishing Group Ltd: 54–60\. doi:10.1136/emj.20.1.54. PMC 1726024. PMID 12533370.
4. ^ a b "Prospective vs. Retrospective Studies". StatsDirect. Archived from the original on 2 February 2012. Retrieved 2 August 2017.
5. ^ a b c d e "Retrospective study". Archived from the original on 2020-02-05. Retrieved 2011-09-02.
6. ^ a b c d e f g Hyde, James N. "Retrospective Cohort Study: Strengths and Weaknesses". Tufts Open Courseware. Tufts University. Archived from the original on 12 May 2017. Retrieved 2 August 2017.
7. ^ "4. The epidemiological approach to investigating disease problems". Archived from the original on 2001-05-05.
8. ^ "Lesson 9: Cohort Study Design; Sample Size and Power Considerations for Epidemiologic Studies". Archived from the original on May 1, 2010.

### Sources

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* This article incorporates public domain material from Dictionary of Cancer Terms. U.S. National Cancer Institute.

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