Cohort Study (Prospective and Retrospective)
Definition: In a cohort study, an investigator selects a group of non-diseased people and follows them over time to determine if they develop a disease/outcome. The cohort is selected based on exposure status, including both people who have been exposed and those who have not. The main characteristic in a cohort study is that the study proceeds from cause to effect.
A cohort study can be prospective or retrospective. In either case, the subjects are selected according to whether they are exposed or not exposed to the factor under investigation, and their subsequent disease status is ascertained. The feature that distinguishes a prospective from a retrospective cohort is simply and solely whether the outcome of interest has occurred at the time the investigator initiates the study. A prospective cohort study is one in which a group of people is followed over time to see if they acquire a disease/outcome. A retrospective study is one in which the disease status of a cohort of people is known." (Epidemiology in Medicine, Hennekens and Buring, 1987)
Associations between exposures and outcomes are measured using relative risk calculations.
Advantages to a Cohort Study
- Results from a cohort study can be used to calculate incidence
- Results from a cohort study can be used to calculate prevalence
- Cohort studies are great for common diseases
- Cohort studies can study multiple diseases/outcomes
Disadvantages to a Cohort Study
- Expensive
- Time consuming
- Requires prohibitively large sample size to detect occurrence of rare diseases
Example 1 (Prospective Study): The water source in your town is contaminated with radioactive material during a terrorist attack. A prospective cohort study is conducted, and the cohort is followed over time to determine if any illnesses occur in people who were exposed compared to people who were not exposed. Relative risk needs to be calculated in order to determine the presence of an association between the radioactive material and the illness.
Example 2 (Retrospective Study): Recently in Mudville, a train derailment resulted in a large chlorine spill. It is suspected the spill is having a residual impact on the residents of Mudville. It is necessary to know if people living within radius "X" of the spill have a higher risk of respiratory symptoms than those living outside of "X." A random sample of town residents is categorized into an exposed group (within the "X" radius) and an unexposed group (outside the "X" radius). Once they are categorized based on exposure status, they are asked about various respiratory symptoms they may be experiencing. Relative risk is calculated to determine if proximity to the spill is associated with current respiratory symptoms.
