Cohort study
A cohort study, sometimes called a panel study, is a form of longitudinal study used in medicine and social science.
A cohort is a group of people who share a common characteristic or experience within a defined period (e.g., are born, leave school, lose their job, are exposed to a drug or a vaccine, etc.). Thus a group of people who were born on a day or in a particular period, say 1948, form a birth cohort. Some cohort studies track groups of children from their birth, and record a wide range of information (exposures) about them. The value of a cohort study depends on the researchers' capacity to stay in touch with all members of the cohort. Some of these studies have continued for decades. Subgroups within the cohort may be compared with each other.
Application
A cohort study is often undertaken to measure the association between a risk factor and a disease. Crucially, the cohort is identified before the appearance of the disease under investigation. The cohort is observed over time to determine the frequency of new incidence of the studied disease.
An example of an epidemiological question that can be answered by the use of a cohort study is: does exposure to X (for example, smoking) correlate with outcome Y (for example, lung cancer)? Such a study would recruit a cohort that contains both smokers and non-smokers. The investigators then follows the cohort for a set period of time and notes differences in the incidence of lung cancer between the smokers and non-smokers. The groups are matched statistically 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).
Classification
Prospective cohort
An example of a cohort study that has been going on for more than 50 years is the Framingham Heart Study.
The largest cohort study in women is the Nurses' Health Study. Started in 1976, it is tracking over 120,000 nurses and has been analyzed for many different conditions and outcomes.
Retrospective cohort
A "prospective cohort" defines the groups before the study is done, while a "retrospective cohort" does the grouping after the data is collected. Whereas prospective cohorts should be summarized with the relative risk, retrospective cohorts should be summarized with the odds ratio. An example of a retrospective cohort is Long-Term Mortality after Gastric Bypass Surgery.[1]
Nested case-control study
An example of a nested case-control study is Inflammatory markers and the risk of coronary heart disease in men and women which was a case control analysis extracted from the Framingham Heart Study cohort.[2]
Alternative study designs
Rare outcomes, or those that slowly develop over long periods, are generally not studied with the use of a cohort study, but are rather studied with the use of a case-control study.
Randomized controlled trials (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 quite unethical.
References
- ↑ Adams TD, Gress RE, Smith SC, et al (2007). "Long-term mortality after gastric bypass surgery". N. Engl. J. Med. 357 (8): 753-61. DOI:10.1056/NEJMoa066603. PMID 17715409. Research Blogging.
- ↑ Pai JK, Pischon T, Ma J, et al (2004). "Inflammatory markers and the risk of coronary heart disease in men and women". N. Engl. J. Med. 351 (25): 2599-610. DOI:10.1056/NEJMoa040967. PMID 15602020. Research Blogging.