Apr 01, 2014, By: Maher M. El-Masri, RN, PhD
Retrospective cohort study design: An observational epidemiologic design in which risk of disease is retrospectively compared between an exposed and a non-exposed group
Source: Gordis, L. (2009). Epidemiology. (4th ed.). Philadelphia: Saunders Elsevier
Retrospective cohort studies are a type of observational research in which the investigator looks back in time at archived or self-report data to examine whether the risk of disease was different between exposed and non-exposed patients. Like prospective cohort studies retrospective cohort studies classify study participants on the basis of whether or not they were exposed to the factor under investigation. However, in retrospective studies, the research is initiated after both the exposure and the outcome (e.g., disease) have already occurred. Therefore, investigators usually create two groups of people who are known to have been either exposed or not exposed to the factor of interest during a specific time frame, and then they compare the two groups with regard to their disease status.
The interpretation of the findings from retrospective cohort studies is similar to that of findings from prospective cohort studies. If the risk of disease in the two groups was not different, we conclude that there is no association between the exposure and the disease. If the risk of disease was higher (or lower) in the exposed group, we conclude that the exposure is associated with increased (or decreased) risk of the disease.
Retrospective cohort studies are often conducted before prospective cohort studies to validate evidence gathered using weaker study designs (e.g., descriptive studies) concerning the association between the exposure and the outcome. They are also commonly carried out when there is a disease outbreak, at which time investigators may look into data recently collected within the context of public health surveillance of the outbreak to establish an association between possible exposures and disease. Finally, the retrospective cohort design is particularly appealing to investigators who may have insufficient funding to conduct a prospective cohort study: because they use archived data, retrospective cohort studies do not require the long, and potentially expensive, followup that prospective cohort studies often require.
Given the retrospective nature of this study design, investigators will always have prior knowledge of the exposure and disease among participants when they begin their research and thus such studies are especially prone to selection bias. Recall bias is also of concern because all events examined in such studies are past events. In addition, retrospective cohort studies are prone to information or misclassification bias because the investigators may not be able to ascertain the accuracy of the archived data concerning the exposure and/or the outcome. Like other observational studies, retrospective cohort studies may only suggest an association between the exposure and the outcome: it is not possible to infer from such studies whether or not the exposure causes the outcome.
In conclusion, the retrospective cohort study design is popular among investigators who wish to conduct a study quickly at low cost, but readers should keep the limitations of this design in mind when they evaluate the findings from such studies.
NurseONE resources on this topic
- Fitzpatrick, J. J., & Kazer M. W. (Eds.). (2011).Encyclopedia of nursing research (3rd ed.).
- Gallin, J. I., & Ognibene, F. P. (Eds.). (2012). Principles and practice of clinical research (3rd ed.).
- Supino, P. G., & Borer, J. S. (Eds.). (2012). Principles of research methodology: A guide for clinical investigators.
Maher M. El-Masri, RN, PhD, is a full professor and research chair in the faculty of nursing, University of Windsor, in Windsor, Ont.