Prospective cohort study
A prospective cohort study is a cohort study that follows over time a group of similar individuals (cohorts) who differ with respect to certain factors under study, to determine how these factors affect rates of a certain outcome. For example, one might follow a cohort of middle-aged truck drivers who vary in terms of smoking habits, to test the hypothesis that the 20-year incidence rate of lung cancer will be highest among heavy smokers, followed by moderate smokers, and then nonsmokers.
The prospective study is important for research on the etiology of diseases and disorders in humans because for ethical reasons people cannot be deliberately exposed to suspected risk factors in controlled experiments.
The distinguishing feature of a prospective cohort study is that at the time that the investigators begin enrolling subjects and collecting baseline exposure information, none of the subjects have developed any of the outcomes of interest. After baseline information is collected, subjects in a prospective cohort study are then followed "longitudinally," i.e. over a period of time, usually for years, to determine if and when they become diseased and whether their exposure status changes. In this way, investigators can eventually use the data to answer many questions about the associations between "risk factors" and disease outcomes. For example, one could identify smokers and non-smokers at baseline and compare their subsequent incidence of developing heart disease. Alternatively, one could group subjects based on their body mass index (BMI) and compare their risk of developing heart disease or cancer. Prospective cohort studies are typically ranked higher in the hierarchy of evidence than retrospective cohort studies and can be more expensive than a case–control study.
One of the advantages of prospective cohort studies is they can help determine risk factors for being infected with a new disease because they are a longitudinal observation over time, and the collection of results is at regular time intervals, so recall error is minimized. 
- "Definition of prospective cohort study - NCI Dictionary of Cancer Terms".
- LaMorte, Wayne. "Prospective and Retrospective Cohort Studies". Boston University College of Public Health. Retrieved Nov. 25, 2013.
- Manolio TA, Bailey-Wilson JE, Collins FS (October 2006). "Genes, environment and the value of prospective cohort studies". Nat. Rev. Genet. 7 (10): 812–20. doi:10.1038/nrg1919. PMID 16983377.
- Porta M (editor). A dictionary of epidemiology. 5th. edition. New York: Oxford University Press, 2008. Edited by Miquel Porta 
This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".Template:Hoang Trong Sang Medical research studies
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