In cohort studies of the role of a suspected factor in the etiology of a disease, it is essential that:
a. There be equal numbers of persons in both study groups
b. At the beginning of the study, those with the disease and those without the disease have equal risks of having the factor
c. The study group with the factor and the study group without the factor be representative of the general population
d. The exposed and nonexposed groups under study be as similar as possible with regard to possible confounding factors
e. Both b and c
Answer: d. The exposed and nonexposed groups under study be as similar as possible with regard to possible confounding factors