A psychiatrist is interested in determining if conditions of individuals who are clinically depressed improve with either 20 or 40 milligrams of Prozac. She administers 20 milligrams to a random half of her clients and 40 milligrams to the other half. After six months, she finds that the clients who took 40 milligrams of Prozac are significantly less depressed than those clients who took 20 milligrams of Prozac. Which type of study did the psychiatrist conduct?
A. correlational research
B. experimental research
C. controlled research
D. hypothetical research