An epidemiologic experiment is performed in which one group is exposed to a suspected factor and the other is not. All individuals with an odd hospital admission number are assigned to the second group. The main purpose of this procedure is to:
a. ensure a double-blind study.
b. prevent observer bias with respect to the factor.
c. prevent observer bias with respect to the outcome.
d. improve the likelihood that the two groups will be comparable with regard to known and unknown confounding factors.
e. guarantee comparability of the two groups with regard to other relevant factors.