Cohort studies. The cohort, or longitudinal, study is an epidemiologic study that follows a defined group, or cohort, over time. It can test hypotheses about whether an exposure to a specific agent is related to the development of disease and can examine multiple disease outcomes that may be associated with exposure to a given agent. A cohort study starts with people who are free of a disease (or other outcome) and classifies them according to whether or not they have been exposed to the agent under study. A cohort study compares health outcomes in individuals who have been exposed to the agent in question with those without the exposure. Such a comparison can be used to estimate a risk difference or a relative risk, two statistics that measure association. The risk difference is the rate of disease in exposed persons minus the rate in unexposed subjects. It represents the absolute number of extra cases of disease associated with the exposure. The relative risk or risk ratio is determined by dividing the rate of developing the disease in the exposed group by the rate in the nonexposed group. A relative risk greater than 1 suggests a positive association between exposure and disease onset. The higher the relative risk, the stronger is the association.

One major advantage of a cohort study is the ability of the investigator to control the classification of subjects at the beginning of the study. This classification in prospective cohort studies is not influenced by the presence of disease because the disease has yet to occur, which reduces an important source of potential bias known as selection bias (see later discussion). A cohort study design also gives the investigator the advantage of measuring and correcting another potential source of bias—confounding. As explained in the next section, when it is possible to measure a confounding factor,4 the investigator can apply statistical methods to minimize its influence on the results. Another advantage of a cohort study is that it is possible to calculate absolute rates of disease incidence.5 A final advantage, especially over cross-sectional studies (discussed below), is that it may be possible to adjust each subject’s follow-up health status for baseline health status so that the person acts as his or her own control, that may reduce a source of variation and increase the power to detect effects. The disadvantages of cohort studies are high costs as a result of a large study population and prolonged periods of follow-up (especially if the disease is rare), attrition of study subjects, and delay in obtaining results.

A prospective cohort study selects subjects on the basis of exposure (or lack of it) and follows the cohort into the future to determine the rate at which the disease (or other health outcome) develops. A retrospective (or historical) cohort study differs from a prospective study in terms of temporal direction; the


A confounding factor is a variable that is independently associated with the health outcome and may affect the results of the study because it is distributed differently in the study and control groups.


Incidence is the rate of occurrence of new cases of an illness or disease in a given population during a specified period of time. Prevalence is the number of cases of an illness or disease existing in a given population at a specific point or period in time.

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