Retrospective cohort study

A retrospective cohort study, also called a historic cohort study, is a longitudinal cohort study that studies a cohort of individuals that share a common exposure factor to determine its influence on the development of a disease, and are compared to another group of equivalent individuals that were not exposed to that factor. Retrospective cohort studies have existed for approximately as long as prospective cohort studies.

It is a medical and psychological research study in which the 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 up with patients over a long period of time (as is the case in a prospective cohort 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. These groups are then assessed retrospectively in order to establish the most likely temporal sequence of events leading up to the current disease state in both the exposed and unexposed groups.

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.

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