Retrospective cohort studies are characterized by all of the following except?

• A. The study groups are exposed and nonexposed
• B. Incidence rates may be computed
• C. They are useful for rare exposures
• D. The required sample size is smaller than that needed for a prospective cohort study

Answer: (D)

Retrospective cohort studies use data already collected in the past to define who was exposed and who was not, then look at the outcomes that occurred over that past time period. The groups are formed based on past exposure status and are followed forward (in the record) to see who develops the outcome, which makes comparisons between exposed and nonexposed individuals meaningful. Because the exposure, the follow-up period, and the outcome events are recorded in existing data, you can calculate incidence or risk within the cohort. These designs are well suited for studying rare exposures, since large historical databases can provide enough exposed individuals for comparison even when the exposure itself is uncommon. The idea that the required sample size is inherently smaller than for a prospective cohort isn’t guaranteed, though; sample size depends on how much data is available and the frequency of the outcome, so retrospective studies can require as large a sample as prospective ones in some situations. So the statement about a smaller required sample size is not a defining feature of retrospective cohort studies.