A psychiatrist tells you she is interested in determining if clinically depressed individuals' condition improves more so with 20 or 40 milligrams of Prozac. She decides to administer 20 milligrams to a random half of her clients who have been diagnosed with depression, and 40 milligrams to the other half of her clients who have been diagnosed with depression. She finds that after 6 months, the clients who took 40 milligrams of Prozac are significantly less depressed than those clients who took 20 milligrams of Prozac. What type of study is the psychiatrist conducting?