with exposure to a given agent (or, for example, to deployment). A cohort study starts with people who are free of a disease (or other outcome) and classifies them according to whether they have been exposed to the agent of interest. It compares health outcomes in people who have been exposed to the agent in question with those who have not.

Cohort studies can be prospective or retrospective. In a prospective cohort study, investigators select a group of subjects and determine who has been exposed and who has not been exposed to a given predictor (independent) variable. They then follow the cohort to determine the rate or risk of the disease (or other health outcome) in the exposed and comparison groups.

A retrospective (or historical) cohort study differs from a prospective study in temporal direction; investigators look back to classify past exposures in the cohort and then track the cohort forward to ascertain the rate of disease.

Cohort studies can be used to estimate a risk difference or a relative risk, two statistics that measure association between the exposure groups. The risk difference, or attributable risk, is the rate of disease in exposed persons minus the rate in unexposed persons, representing the number of extra cases of disease attributable to the exposure. The relative risk is determined by dividing the rate of those who develop the disease in the exposed group (for example, the deployed group) by the rate of those developing the disease in the nonexposed group (for example, the nondeployed group). A relative risk greater than 1 suggests an association between exposure and disease onset; the higher the relative risk, the stronger the association.

Cohort studies have several advantages and disadvantages. Generally, the advantages outweigh the disadvantages if the study is well designed and executed. The advantages of cohort studies include the following:

• The investigator knows that the predictor variable preceded the outcome variable.

• Exposure can be defined and classified at the beginning of the study and subjects can be selected based on exposure definition.

• Information on potential confounding variables can be collected in a prospective cohort study so that they may be controlled in the analysis.

• Rare or unique exposures (such as Gulf War exposures) can be studied, and the investigators can study multiple health outcomes.

• Absolute rates or risk of disease incidence and prevalence can be calculated.¹

Disadvantages of cohort studies include the following:

• They are often expensive because of the long periods of followup (especially if the disease has a delayed onset, for example, cancer), attrition of study subjects, and delay in obtaining results.

• They are inefficient for the study of rare diseases or diseases of long latency.