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Cohort study
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==Comparison with controlled trials== Cohort studies differ from clinical trials in that no intervention, treatment, or exposure is administered to participants in a cohort design; and no control group is defined. Rather, cohort studies are largely about the life histories of segments of populations and the individual people who constitute these segments.<ref>{{cite web|url=http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html|title=Cohort Studies|publisher=Web Center for Social Research Methods|archive-url=https://web.archive.org/web/20110909043451/http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html|archive-date=2011-09-09}}</ref><ref>{{cite book|url=http://www.oup.com/us/catalog/general/subject/Medicine/EpidemiologyBiostatistics/?view=usa&ci=9780195314502|title=A Dictionary of Epidemiology|publisher=Oxford University Press|year=2008|editor-last=Porta|editor-first=Miquel|edition=5th|location=New York}}</ref> Exposures or protective factors are identified as preexisting characteristics of participants. The study is controlled by including other common characteristics of the cohort in the statistical analysis. Both exposure/treatment and control variables are measured at baseline. Participants are then followed over time to observe the incidence rate of the disease or outcome in question. Regression analysis can then be used to evaluate the extent to which the exposure or treatment variable contributes to the incidence of the disease, while accounting for other variables that may be at play.{{cn|date=May 2023}} [[Randomized controlled trial|Double-blind randomized controlled trial]]s (RCTs) are generally considered superior methodology in the [[hierarchy of evidence]] in treatment, because they allow for the most control over other variables that could affect the outcome, and the randomization and blinding processes reduce bias in the study design. This minimizes the chance that results will be influenced by [[confounding]] variables, particularly ones that are unknown. However, educated hypotheses based on prior research and background knowledge are used to select variables to be included in the regression model for cohort studies, and statistical methods can be used to identify and account for potential confounders from these variables. Bias can also be mitigated in a cohort study when selecting participants for the cohort. RCTs may not be suitable in all cases; such as when the outcome is a negative health effect and the exposure is hypothesized to be a risk factor for the outcome. Ethical standards, and morality, would prevent the use of risk factors in RCTs. The natural or incidental exposure to these risk factors (e.g. time spent in the sun), or self-administered exposure (e.g. smoking), can be measured without subjecting participants to risk factors outside of their individual lifestyles, habits, and choices.
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