National EMSC Data Analysis Resource Center
In a retrospective study, the outcome of interest has already occurred at the time the study is initiated. A retrospective study design allows the investigator to formulate ideas about possible associations and investigate potential relationships, although causal statements usually should not be made.
An investigator conducting a retrospective study typically utilizes administrative databases, medical records, or interviews with patients who are already known to have a disease or condition. In general, the reasons to conduct a retrospective study are to:
We’ll discuss two types of retrospective studies:
In a retrospective case-control study the investigator can quickly estimate the effect of an exposure on outcome status. Cases and controls are established based on the presence of the condition, and exposure is assessed by looking back over time. It is very important in a case-control study that the cases be as similar to the controls on all factors except the outcome of interest.
A retrospective cohort study allows the investigator to describe a population over time or obtain preliminary measures of association to develop future studies and interventions. The exposure and outcome information in a cohort study are identified retrospectively by using administrative datasets, reviewing patient charts, conducting interviews, etc.