A psychiatrist tells you that she is interested in determining if the condition of individuals who are clinically depressed improves with either 20 or 40 milligrams of Prozac. She decides to administer 20 milligrams to a random half of her clients, and 40 milligrams to the other half. She finds that after 6 months the clients who took 40 milligrams are significantly less depressed then those clients who took 20 milligrams of Prozac. Which type of research didi the psychiatrist conduct?