An observational study in which the parameters of a group of subjects exposed to a given factor are compared with those of a similar group of subjects not exposed to the factor.
Note: To make the groups comparable, the investigators may match subjects exposed to a given factor with subjects not exposed to the factor on the basis of common characteristics (e.g. age, gender and disease severity) or may make the necessary statistical adjustments to obtain, in the matched subjects, characteristics that are as similar as possible.
Other definition: An observational study in which the investigators select and monitor a group (cohort) of subjects exposed and non exposed to a given factor, to verify the effects of the exposure to the factor on the health of the cohort’s members.
Note: This type of study is usually prospective, but may also be retrospective when the data have already been gathered. What is important is the starting point: identify the subjects exposed and not exposed to a given factor and then determine the health effects of the exposure.