A study that looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is already established before the study commences. Retrospective studies rely on data (e.g., about nutritional behavior and developed diseases) that have already been collected in the past or need to be recalled by the participants. Case-control studies are usually retrospective, cohort studies sometimes are, randomized controlled trials never are.
Retrospective studies are considered to be of comparable poor quality as the risk of bias (e.g., inaccurate recollection of past behavior) is high. Sources of bias are more common in retrospective studies than in prospective studies.