Feb 20, 2014
Terminology 101: Prospective cohort study design
Prospective cohort study design: An observational epidemiologic design in which an exposed group and a non-exposed group are followed to compare their incidence of disease
Source: Gordis, L. (2009). Epidemiology. (4th ed.). Philadelphia: Saunders Elsevier
A cohort is a group of individuals who share an attribute or experience (e.g., pregnant women, patients treated in intensive care units or schoolchildren). Prospective cohort studies are a type of observational research in which the investigator examines whether or not exposure to a certain factor is associated with subsequent development of disease. Specifically, the investigator follows a group of exposed individuals and a group of non-exposed individuals over time to compare their risk of developing disease.
Although the exposure status of study participants is often known at the beginning of a prospective cohort study, this is not always the case. For instance, a researcher who wants to study the association between having a chest tube (the exposure) and developing bacteremia (the outcome) in ICU patients may recruit patients as they are admitted to the ICU, without knowing in advance which of them will have a chest tube inserted during their ICU stay. Patients who end up having a chest tube during the course of the study will then be compared with patients who do not end up having a chest tube with regard to their risk of bacteremia.
In a prospective cohort study, if the risk of disease in the non-exposed group is the same as that in the exposed group, we conclude that there is no association between the exposure and the disease. If the risk of disease is higher (or lower) in the exposed group, we conclude that the exposure is associated with increased (or decreased) risk of the disease. These associations are often established by calculating the relative risk (see the April 2013 column) or odds ratio (see the June 2013 column).
A major advantage of prospective cohort studies is that patients are followed into the future, making it possible to ascertain the temporal relationship between the exposure and the outcome (i.e., making it possible to ascertain that the exposure took place before the outcome). Following patients into the future also minimizes the likelihood of recall bias because data on the exposure and the outcome are collected in real time. A third advantage of prospective cohort studies is that the impact of an exposure on multiple outcomes can be examined in a single study. However, losing contact with patients over the course of the study, referred to as loss to followup, is a major disadvantage of this type of study, particularly when long followup periods are involved. Finally, as in all observational studies, one cannot rule out the possibility of selection bias or determine that the exposure causes the outcome: one can only infer that there is an association between the exposure and the outcome.
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.