The most important cohort studies are those that track a group of children from their birth, and record a wide range of information 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.
Shorter term cohort 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 a endpoint or outcome measure between the two groups.
For example, if one hypothesises that exposure to X (say, smoking) causes outcome Y (say, lung cancer), a cohort study is one means of testing this hypothesis. Such a study would enrol a group of smokers and a group of non-smokers (the control group) and follow them for a set period (say 20 years) and note differences in the incidence of lung cancers between the groups at the end of this time. In this example, a statistically significant increase in the incidence of lung cancers in the smoking group as compared to the non-smoking group is evidence in favour of our hypothesis.
Clearly the choice of control group will influence the reliability of the results. If, for example the majority of the non-smoking group had a history of asbestos exposure (a known risk facter for lung cancers), which the smoking group did not, and the study found no significant difference in the incidence of cancer between the groups, then we have reason to doubt the reliability of this negative result. A positive result may have been masked by an increased incidence of lung cancers, due to asbestos exposure, in the control group. For this reason, the groups are usually matchedfor a number of baseline variables, such as age, sex, race, socioeconomic factors, and other confounding variables, such as asbestos exposure in this case. This may be difficult to achieve, when for example, lower socioeconic status is associated with increased smoking prevalence. Nevertheless, the possibility remains that variables that have not been considered could be confounding results.
Randomized controlled trials, or RCTs are a superior methodology in the hierarchy of evidence, because they limit the potential for bias by randomly assigning the one patient pool to intervention or control groups. This minimises the chance that the incidence of confounding variables will differ between the two groups (e.g. the smoking-socioeconomic status association mentioned above).
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!
from: http://www.knowledgerush.com/kr/encyclopedia/Cohort_study/
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