Editing Cohort study (section)

==Examples==
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An example of an epidemiological question that can be answered using a cohort study is whether exposure to X (say, smoking) associates with outcome Y (say, lung cancer). For example, in 1951, the [[British Doctors Study]] was started. Using a cohort which included both smokers (the exposed group) and non-smokers (the unexposed group). The study continued through 2001. By 1956, the study provided convincing proof of the association between smoking and the incidence of lung cancer. In a cohort study, the groups are ''matched'' in terms of many other variables such as economic status and other health status so that the variable being assessed, the [[independent variable]] (in this case, smoking) can be isolated as the cause of the [[dependent variable]] (in this case, lung cancer). In this example, a [[Statistical significance|statistically significant]] increase in the incidence of lung cancer in the smoking group as compared to the non-smoking group is evidence in favor of the hypothesis. However, rare outcomes, such as lung cancer, are generally not studied with the use of a cohort study, but are rather studied with the use of a [[case-control]] study.

In the Arts, Cohort Succession can explain most change in literature, art, intellectualism, political opinions, and phonology.<ref>{{cite journal |last1=Underwood |first1=Ted |last2=Kiley |first2=Kevin |last3=Shang |first3=Wenyi |last4=Vaisey |first4=Stephen |title=Cohort Succession Explains Most Change in Literary Culture |journal=Sociological Science |date=2022 |volume=9 |pages=184–205 |doi=10.15195/v9.a8 |url=https://sociologicalscience.com/articles-v9-8-184/ |access-date=11 May 2022 |doi-access=free }}</ref>

Shorter term studies are commonly used in medical research as a form of [[clinical trial]], or means to test a particular hypothesis of clinical importance. Such studies typically follow two groups of patients for a period of time and compare an endpoint or outcome measure between the two groups.

[[Randomized controlled trial]]s, or RCTs, are a superior methodology in the hierarchy of evidence, because they limit the potential for bias by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimizes the chance that the incidence of confounding variables will differ between the two groups. Nevertheless, it is sometimes not practical or ethical to perform RCTs to answer a clinical question. To take our example, if we already had reasonable evidence that smoking causes lung cancer then persuading a pool of non-smokers to take up smoking in order to test this hypothesis would generally be considered unethical.

Two examples of cohort studies that have been going on for more than 50 years are the [[Framingham Heart Study]] and the [[National Child Development Study]] (NCDS), the most widely researched of the [[British birth cohort studies]]. Key findings of NCDS and a detailed profile of the study appear in the ''International Journal of Epidemiology''.<ref>{{cite journal |vauthors=Power C, Elliott J|title=Cohort profile: 1958 British Cohort Study |journal=International Journal of Epidemiology |volume=35 |issue=1 |pages=34–41 |year=2006 |doi=10.1093/ije/dyi183 |pmid=16155052|doi-access=free }}</ref>

The [[Dunedin Multidisciplinary Health and Development Study]] has been studying the thousand people born in [[Dunedin]], New Zealand, in 1972–1973. The subjects are interviewed regularly, with Phase (age) 52 starting in 2024.

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The ''International Journal of Epidemiology''<ref>{{cite journal |first1=Richard J |last1=Pinder |first2=Neil |last2=Greenberg |first3=Edward J |last3=Boyko |first4=Gary D |last4=Gackstetter |first5=Tomoko I |last5=Hooper |first6=Dominic |last6=Murphy |first7=Margaret AK |last7=Ryan |first8=Besa |last8=Smith |first9=Tyler C |last9=Smith |first10=Timothy S |last10=Wells |first11=Simon |last11=Wessely |title=Profile of two cohorts: UK and US prospective studies of military health   |journal=[[International Journal of Epidemiology]] |year=2012 |volume=41 |number=5 |pages=1272–1282 |publisher=[[Oxford University Press]] |doi=10.1093/ije/dyr096}}</ref> comparison of two Cohorts, [[Millennium Cohort Study (United States)]] and The King's Cohort (United Kingdom)

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The largest cohort study in women is the [[Nurses' Health Study]]. Starting in 1976, it is tracking over 120,000 nurses and has been analyzed for many different conditions and outcomes.

The largest cohort study in Africa is the [[Birth to Twenty]] Study, which began in 1990 and tracks a cohort of over 3,000 children born in the weeks following [[Nelson Mandela]]'s release from prison.

Other famous examples are the [[Grant Study]] tracking a number of Harvard graduates from ca. 1950.77, the [[Whitehall Study]] tracking 10,308 British civil servants, and the [[Caerphilly Heart Disease Study]], which since 1979 has studied a representative sample of 2,512 men, drawn from the Welsh town of Caerphilly.<ref>{{Cite journal|pmc=1052363|year=1984|title=Caerphilly and Speedwell collaborative heart disease studies. The Caerphilly and Speedwell Collaborative Group|journal=Journal of Epidemiology and Community Health|volume=38|issue=3|pages=259–262|pmid=6332166|doi=10.1136/jech.38.3.259}}.</ref>

The ASPREE-XT study is designed to determine whether there are long-lasting effects of an average of four–five years of treatment with daily low-dose aspirin, with outcome measures including cancer mortality.<ref>{{Cite web|url=https://aspree.org/aus/for-clinicans/about-aspree-xt/|title=ASPREE Australia {{!}} ABOUT ASPREE-XT - ASPREE Australia|date=2019-07-21|access-date=2019-07-21|archive-url=https://web.archive.org/web/20190721044912/https://aspree.org/aus/for-clinicans/about-aspree-xt/|archive-date=21 July 2019}}</ref> As of September 2018, there were 16,703 ASPREE-XT participants in Australia.<ref>{{Cite web|date=September 16, 2018|title=Daily low-dose aspirin found to have no effect on healthy life span in older people|url=https://www.nih.gov/news-events/news-releases/daily-low-dose-aspirin-found-have-no-effect-healthy-life-span-older-people|access-date=22 March 2021|website=National Institutes of Health News Releases}}</ref> It has been proposed that the existing ASPREE-XT study could provide a platform for a future multigenerational research study.<ref>{{Cite journal|last1=Nunn|first1=Jack S.|last2=Sulovski|first2=Merrin|last3=Tiller|first3=Jane|last4=Holloway|first4=Bruce|last5=Ayton|first5=Darshini|last6=Lacaze|first6=Paul|date=2021-05-03|title=Involving elderly research participants in the co-design of a future multi-generational cohort study|journal=Research Involvement and Engagement|volume=7|issue=1|pages=23|doi=10.1186/s40900-021-00271-4|pmid=33941290|pmc=8094476|issn=2056-7529|doi-access=free}}</ref>