Retrospective cohort study
A retrospective cohort study, also called a historic cohort study, 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 order to differentiate Retrospective versus Prospective Cohort, following analogy can be considered.
A retrospective (historic) cohort study is different from a prospective cohort study in the manner in which it is conducted. In case of Retrospective Cohort Study, the investigator basically 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 Nonexposed; 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 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.
As is obvious, Retrospective Cohort has the benefits of being cheaper and less time consuming. The resources are mainly directed at collection of data only. Additionally, it has essentially all the benefits of a Cohort Study (Statistics)
This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".
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