Retrospective Cohort Studies
The design of a retrospective cohort study is essentially the same as that of a prospective cohort study: a group of subjects is followed over time with measurements of potential predictor variables at the beginning and then asertainment of subsequent out
The difference is that the assembly of the cohort, baseline measurements, follow-up, and outcomes all happened in the past.
Only possible if adequate data is available on a cohort subjects which has been assembled for other purposes.