One hundred volunteers who suffer from agoraphobia are available for a study. Fifty are selected at random and are given the drug imipramine, which is believed to be effective in treating agoraphobia. The other 50 are given a placebo. A psychiatrist evaluates the symptoms of all volunteers after two months to determine if there has been substantial improvement in the severity of the symptoms.
Assume the volunteers were first divided in men and women, and then half of the men were randomly assigned to the new drug and half of the women were assigned to the new drug. The remaining volunteers received the placebo. This would be an example of
A. Replication.
B. Confounding. The effects of gender will be mixed up with the effects of the drugs.
C. A block design.
D. A matched pairs design.