ABSTRACT
A cohort study can be either prospective (current) or retrospective (historical). In a prospective study, the data on exposure are collected prior to the occurrence of disease and subjects are followed up over time to observe occurrence of the disease. In a retrospective study, past exposure data are assembled for a defined cohort using historical records. The disease outcome may or may not be known at this time; ideally it should be obtained independent from or subsequent to assembly of exposure data.
