Retrospective cohort study

[1] 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.

[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.

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.