In contrast with case-control studies, cohort studies
evaluate the frequency of illness in a group following
an identified exposure compared to the frequency of
illness in a group of unexposed controls. A Cohort study
layout shown in Table 4. In contrast with a case-control
study (which allows evaluation of multiple exposures
possibly linked to a given disease), the cohort study
allows evaluation of multiple disease outcomes for a
single exposure. Common measures of illness used in cohort studies of
chronic disease are cancer incidence and cause-specific
mortality. The majority of chronic disease studies employ
mortality as the primary study outcome, but the incidence
data are preferrable when available. For cancers with a
low survival rate, such as lung cancer, the 2 outcomes
may be roughly equivalent. For other cancers, such as
prostate cancer or breast cancer, mortality may more
strongly reflect access to medical care than disease incidence.
