{{Short description|Historic cohort study}} 360px|thumb|upright=1|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.<ref>{{Cite web|url=http://www.epidemiology.ch/history/papers/doll-paper2.pdf|title=What is epidemiology?|date=11 March 2019|access-date=29 August 2011|archive-date=12 August 2017|archive-url=https://web.archive.org/web/20170812143315/http://www.epidemiology.ch/history//papers/doll-paper2.pdf|url-status=live}}</ref>
==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 (such as lung cancer).<ref name="urlDefinition of historic cohort study - NCI Dictionary of Cancer Terms">{{Cite web|url=http://www.cancer.gov/dictionary?CdrID=286525|title=Definition of historic cohort study - NCI Dictionary of Cancer Terms }}</ref> 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<ref name=Mann/> to determine the relative risk of the cohort compared to the control group.<ref name="statsdirect.com">{{cite web|title=Prospective vs. Retrospective Studies|url=http://www.statsdirect.com/help/basics/prospective.htm|website=StatsDirect|access-date=2 August 2017|archive-date=2 February 2012|archive-url=https://web.archive.org/web/20120202211826/http://www.statsdirect.com/help/basics/prospective.htm|url-status=live}}</ref>
This is fundamentally the same methodology as for a prospective cohort study, except that the retrospective study is performed post-hoc, looking back.<ref name=Mann>{{cite journal|last1=Mann|first1=C J|title=Observational research methods. Research design II: cohort, cross sectional, and case-control studies|journal=Emergency Medicine Journal|date=1 January 2003|volume=20|issue=1|pages=54–60|url= |publisher=BMJ Publishing Group Ltd|doi=10.1136/emj.20.1.54|pmc=1726024|pmid=12533370}}</ref> The prospective study looks forward, enrolling patients unaffected by the outcome and observing them to see whether the outcome has occurred.<ref name=Mann/> 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.{{cn|date=November 2023}}
Retrospective cohort studies require particular caution because errors due to confounding and bias are more common than in prospective studies.<ref name="statsdirect.com"/>
==Advantages== Retrospective cohort studies exhibit the benefits of cohort studies and have distinct advantages relative to prospective ones: * They are conducted on a smaller scale.<ref name="medical-dictionary.thefreedictionary.com"/> * They typically require less time to complete.<ref name="medical-dictionary.thefreedictionary.com"/> * They are generally less expensive, because resources are mainly devoted to collecting data.<ref name="ocw.tufts.edu"/> * They are better for analyzing multiple outcomes.<ref name="ocw.tufts.edu"/> * In a medical context, they can potentially address rare diseases, which would necessitate extremely large cohorts in prospective studies.<ref name="medical-dictionary.thefreedictionary.com"/> Retrospective studies are especially helpful in addressing diseases of low incidence, since affected people have already been identified so .<ref>{{cite web| url = http://www.fao.org/Wairdocs/ILRI/x5436E/x5436e06.htm| url-status = dead| archive-url = https://web.archive.org/web/20010505220122/http://www.fao.org/Wairdocs/ILRI/x5436E/x5436e06.htm| archive-date = 2001-05-05| title = 4. The epidemiological approach to investigating disease problems}}</ref> The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit.<ref name="ocw.tufts.edu"/> Additionally, it has essentially all the benefits of a cohort study.
==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.<ref name="medical-dictionary.thefreedictionary.com">{{Cite web|url=http://medical-dictionary.thefreedictionary.com/retrospective+study|title=Retrospective study|access-date=2011-09-02|archive-date=2020-02-05|archive-url=https://web.archive.org/web/20200205173026/http://medical-dictionary.thefreedictionary.com/retrospective+study|url-status=live}}</ref> * Researchers cannot control exposure or outcome assessment, and instead must rely on others for accurate recordkeeping.<ref name="ocw.tufts.edu"/> When relying on individual recall of former exposure to risk variables, recall may be inaccurate and subject to biases.<ref name="medical-dictionary.thefreedictionary.com"/> It can be very difficult to make accurate comparisons between the exposed and the non-exposed.<ref name="ocw.tufts.edu"/> * The retrospective aspect may introduce selection bias and mis-classification or information bias.<ref>{{cite web |title=Lesson 9: Cohort Study Design; Sample Size and Power Considerations for Epidemiologic Studies |url=https://onlinecourses.science.psu.edu/stat507/node/54 |archive-url=https://web.archive.org/web/20100501165553/https://onlinecourses.science.psu.edu/stat507/node/54 |publisher=Pennsylvania State University|archive-date=May 1, 2010 |url-status=deviated}}</ref> With retrospective studies, the temporal relationship is frequently difficult to assess.<ref name="ocw.tufts.edu">{{cite web|last1=Hyde|first1=James N|title=Retrospective Cohort Study: Strengths and Weaknesses|url=http://ocw.tufts.edu/Content/1/lecturenotes/194039/194062|website=Tufts Open Courseware|publisher=Tufts University|access-date=2 August 2017|archive-url=https://web.archive.org/web/20170512164859/http://ocw.tufts.edu/Content/1/lecturenotes/194039/194062|archive-date=12 May 2017}}</ref> * Retrospective studies may need very large sample sizes for rare outcomes.<ref name="ocw.tufts.edu"/>
==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.{{cn|date=November 2023}}
==See also== * Case-control study
== References == === Citations === {{Reflist}}
=== Sources === {{refbegin}} * {{NCI-cancer-dict}} {{refend}}
{{-}} {{Medical research studies}}
Category:Cohort study methods Category:Clinical research Category:Nursing research