Cohort study

A cohort study is a form of longitudinal study used in medicine and social science. It is one type of Study design.

In medicine, it is usually undertaken to obtain additional evidence to refute or support the existence of an association between suspected cause and disease. The cohorts are identified prior to the appearance of the disease under investigation. The study groups, so defined, are observed over a period of time to determine the frequency of disease among them. The main characteristic is that the study proceeds from cause to effect.

In epidemiology, the term cohort is defined as a group of people who share a common characteristic or experience within a defined time period (e.g., age, occupation, exposure to a drug or a vaccine, insured persons etc.). Thus a group of people who were born on a day or in a particular time period, say 1948, form a birth cohort. The comparison group may be the general population from which the cohort is drawn, or it may be another cohort of persons thought to have had little or no exposure to the substance under investigation, but otherwise similar.

Some cohort studies track a group 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. An example of a cohort study that has been going on for more than 50 years is the Framingham Heart Study.

An example of an epidemiologic question that can be answered by the use of a cohort study is: does exposure to X (say, smoking) correlate with outcome Y (say, lung cancer)? Such a study would enroll a group of smokers and a group of non-smokers (the unexposed group) and follow them for a set period of time and note differences in the incidence of lung cancer 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 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.

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 trials, 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 minimises 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.

A "prospective cohort" defines the groups before the study is done, while a "retrospective cohort" does the grouping after the data is collected.