A retrospective cohort study, also called a historic cohort study, generally means to take a look back at events that already have taken place. For example, the term is used in medicine, describing a look back at a patient's medical history or lifestyle. Retrospective cohort studies have existed for approximately as long as prospective cohort studies.
Design
It is a medical research study in which the medical records of groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and those who do not smoke) are compared for a particular outcome (such as lung cancer). In retrospective cohort studies, a risk ratio or odds ratio gives an assessment of relative risk.
In the case of a retrospective cohort study, the investigator collects data from past records and does not follow patients up as is the case with a prospective study. However, the starting point of this study is the same as for all cohort studies. The first objective is still to establish two groups - exposed versus non-exposed; and these groups are followed up in the ensuing time period.
In a nutshell, in Retrospective Cohort Study, all the events - exposure, latent period, and subsequent outcome (ex. development of disease) have already occurred in the past. We merely collect the data now, and establish the risk of developing a disease if exposed to a particular risk factor. On the other hand, Prospective Cohort Study is conducted by starting with two groups at the current point, and following up in future for occurrence of disease, if any.
It is important to understand that the methodology of prospective and retrospective cohort studies is fundamentally the same, but the retrospective study is performed post-hoc, as the cohort is followed retrospectively. The time to complete a retrospective study is only as long as it takes to collect and interpret the data. Retrospective studies examine possible risk and protection variables in relation to a result that is already established at the start of the study.
Caution needs to be exercised, in particular, with retrospective cohort studies because errors due to confounding and bias are more common in retrospective studies than in prospective studies.
Advantages
Retrospective cohort studies have the following distinct advantages when compared with prospective cohort studies: they are conducted on a smaller scale, they typically require less time to complete, they are better for analysing multiple outcomes, and, in a medical context, they can potentially address rare diseases, which would necessitate extremely large cohorts in prospective studies. In such a study, diseased people have already been identified so retrospective studies are especially helpful in addressing diseases of low incidence. The fact that retrospective studies are generally less expensive than prospective studies may be another key benefit. These studies tend to be less expensive in part because outcome and exposure have already occurred, and the resources are directed at mainly collection of data. Additionally, it has essentially all the benefits of a Cohort Study (Statistics)
Disadvantages
Retrospective studies have disadvantages vis-a-vis prospective studies. Among the disadvantages are that some key statistics cannot be measured, and significant biases may affect the selection of controls. Additionally, major biases with retrospective cohort studies can impact the recall of former exposure to risk variables. Among the biases which can negatively impact the veracity of this type of study are selection bias and misclassification or information bias as a result of the retrospective aspect. With retrospective studies, the temporal relationship is frequently difficult to assess. Also, those who conduct retrospective studies cannot control exposure or outcome assessment but instead need to rely on others for accurate recordkeeping. That is particularly problematic because it can be very difficult to make accurate comparisons between the exposed and the nonexposed. Retrospective studies also may need very large sample sizes for rare outcomes.
References
 This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".
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