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).[2] 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[3] to determine the relative risk of the cohort compared to the control group.[4]
This is fundamentally the same methodology as for a prospective cohort study, except that the retrospective study is performed post-hoc, looking back.[3] The prospective study looks forward, enrolling patients unaffected by the outcome and observing them to see whether the outcome has occurred.[3] However, both kinds of cohort studies share the same starting point (considering data from before the occurrence of the outcome). 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.[citation needed]
Retrospective cohort studies require particular caution because errors due to confounding and bias are more common than in prospective studies.[4]
Advantages
Retrospective cohort studies exhibit the benefits of cohort studies and have distinct advantages relative to prospective ones:
They are generally less expensive, because resources are mainly devoted to collecting data.[6]
They are better for analyzing multiple outcomes.[6]
In a medical context, they can potentially address rare diseases, which would necessitate extremely large cohorts in prospective studies.[5]
Retrospective studies are especially helpful in addressing diseases of low incidence, since affected people have already been identified so .[7] The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit.[6] Additionally, it has essentially all the benefits of a cohort study.
Disadvantages
Retrospective studies have disadvantages vis-a-vis prospective studies:
Some key statistics cannot be measured, and significant biases may affect the selection of controls.[5]
Researchers cannot control exposure or outcome assessment, and instead must rely on others for accurate recordkeeping.[6] When relying on individual recall of former exposure to risk variables, recall may be inaccurate and subject to biases.[5] It can be very difficult to make accurate comparisons between the exposed and the non-exposed.[6]
The retrospective aspect may introduce selection bias and mis-classification or information bias.[8] With retrospective studies, the temporal relationship is frequently difficult to assess.[6]
Retrospective studies may need very large sample sizes for rare outcomes.[6]
Comparison with case-control studies
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. Both the relative risk and odds ratio are relevant in retrospective cohort studies, but only the odds ratio can be used in case-control studies. Although most case-control studies are retrospective, they can also be prospective when the researcher still enrolls participants based on the occurrence of a disease as new cases occur.[citation needed]
