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. The required sample size is smaller than that needed for a prospective cohort study
d. The required sample size is similar to that needed for a prospective cohort study
e. They are useful for rare exposures
Answer: c. The required sample size is smaller than that needed for a prospective cohort study