Editing Cohort study (section)

===Current and historical cohorts===
{{main|Retrospective cohort study}}

A current cohort study represents a true prospective study where the data concerning exposure are assembled prior to the occurrence of the fact to be studied, for instance a disease. An example of a current cohort study is the Oxford Family Planning Association Study in the United Kingdom, which aimed to provide a balanced view of the beneficial and harmful effects of different methods of contraception. This study has provided a large amount of information on the efficacy and safety of contraceptive methods, and in particular oral contraceptives (OCs), diaphragms and intrauterine device (IUDs).<ref>{{Cite journal|last1=Vessey|first1=M. P.|last2=Lawless|first2=M.|date=1984|title=The Oxford-Family Planning Association contraceptive study|journal=Clinics in Obstetrics and Gynaecology|volume=11|issue=3|pages=743–757|doi=10.1016/S0306-3356(21)00625-7|issn=0306-3356|pmid=6509857}}</ref>

In a historical cohort study the data concerning exposure and occurrence of a disease, births, a political attitude or any other categorical variable are collected after the events have taken place, and the subjects (those exposed and unexposed to the agent under study) are assembled from existing records or health care registers.

A "[[prospective cohort]]" defines the groups before the study is done, while historical studies, which are sometimes referred to as "[[retrospective cohort]]", define the grouping after the data is collected. Examples of a [[retrospective cohort]] are ''Long-Term Mortality after Gastric Bypass Surgery''<ref name="pmid17715409">{{cite journal |author=Adams TD |title=Long-term mortality after gastric bypass surgery |journal=N. Engl. J. Med. |volume=357 |issue=8 |pages=753–61 |year=2007 |pmid=17715409 |doi=10.1056/NEJMoa066603 |name-list-style=vanc|author2=Gress RE |author3=Smith SC |display-authors=3 |last4=Halverson |first4=R. Chad |last5=Simper |first5=Steven C. |last6=Rosamond |first6=Wayne D. |last7=Lamonte |first7=Michael J. |last8=Stroup |first8=Antoinette M. |last9=Hunt |first9=Steven C.|s2cid=8710295 |url=http://pdfs.semanticscholar.org/2908/f0ef315574fb7fa5a7941c97091a7fddd012.pdf |archive-url=https://web.archive.org/web/20190220132224/http://pdfs.semanticscholar.org/2908/f0ef315574fb7fa5a7941c97091a7fddd012.pdf |url-status=dead |archive-date=2019-02-20 }}</ref> and ''[[The Lothian Birth Cohort Studies]]''.<ref>{{cite web|url=http://www.psy.ed.ac.uk/research/lbc/LBC.html|title=The Lothian Birth Cohort Studies|access-date=8 May 2011|publisher=University of Edinburgh |archive-url=https://web.archive.org/web/20091022052040/http://www.psy.ed.ac.uk/research/lbc/LBC.html |archive-date=2009-10-22}}</ref>

Although historical studies are sometimes referred to as retrospective study, it a misnomer as the methodological principles of historical cohort studies and prospective studies are the same.<ref name="gfmer.ch" />