In a study designed to test a new antidepressant, a large gr…

In a study designed to test a new antidepressant, a large group of outpatient psychiatric patients was randomly assigned to one of two groups. One of the groups was given the drug as a pill. The other group was given identical-looking inert pills. All participants were tested in the morning. The level of depression of each subject was measured by three psychologists independently, using the Beck Depression Inventory. Which was the independent variable in this study?