While retrospective cohort studies try to compare the risk of developing a disease to some already known exposure factors, a case-control study will try to determine the possible exposure factors after a known disease incidence. Retrospective studies may need very large sample sizes for rare outcomes.With retrospective studies, the temporal relationship is frequently difficult to assess. The retrospective aspect may introduce selection bias and mis-classification or information bias.It can be very difficult to make accurate comparisons between the exposed and the non-exposed. When relying on individual recall of former exposure to risk variables, recall may be inaccurate and subject to biases. Researchers cannot control exposure or outcome assessment, and instead must rely on others for accurate recordkeeping.Some key statistics cannot be measured, and significant biases may affect the selection of controls.Retrospective studies have disadvantages vis-a-vis prospective studies: Additionally, it has essentially all the benefits of a cohort study. The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit. Retrospective studies are especially helpful in addressing diseases of low incidence, since affected people have already been identified so. In a medical context, they can potentially address rare diseases, which would necessitate extremely large cohorts in prospective studies.They are better for analyzing multiple outcomes.They are generally less expensive, because resources are mainly devoted to collecting data.They typically require less time to complete.They are conducted on a smaller scale.Retrospective cohort studies exhibit the benefits of cohort studies and have distinct advantages relative to prospective ones:

Retrospective cohort studies require particular caution because errors due to confounding and bias are more common than in prospective studies. The first objective is still to establish two groups - exposed versus non-exposed - which are then assessed retrospectively to establish the most likely temporal sequence of events leading to the current disease state in both the exposed and unexposed groups.

However, both kinds of cohort studies share the same starting point (considering data from before the occurrence of the outcome). The prospective study looks forward, enrolling patients unaffected by the outcome and observing them to see whether the outcome has occurred. This is fundamentally the same methodology as for a prospective cohort study, except that the retrospective study is performed post-hoc, looking back. Data on the relevant events for each individual (the form and time of exposure to a factor, the latent period, and the time of any subsequent occurrence of the outcome) are collected from existing records and can immediately be analyzed to determine the relative risk of the cohort compared to the control group. The retrospective cohort study compares groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and ones who do not smoke) in terms of a particular outcome (such as lung cancer).

Types of retrospective studies