In a retrospective study, in contrast to a prospective study, the outcome of interest has already occurred at the time the study is initiated. There are two types of retrospective study: a case–control study and a retrospective cohort study. A retrospective study design allows the investigator to formulate hypotheses about possible associations between an outcome and an exposure and to further investigate the potential relationships. However, a causal statement on this association usually should not be made from a retrospective study.

In conducting a retrospective study, an investigator typically uses administrative databases, medical records, or interviews with patients who are already known to have a disease or condition. Generally, a retrospective study is the method of choice for the study of a rare outcome, for a ...