7 Treating depression with a combination of drugs. Physicians are now experimenting with using a combination of drugs, rather than a single drug, to treat major depression. In a study published in the American Journal of Psychiatry

(March 2010), a sample of 105 patients diagnosed with a major depressive disorder were randomly assigned to one of four drug combination groups. Group 1 received daily doses of the antidepressant drug fluoxetine and a placebo;

group 2 received the antidepressant drug mirtazapine plus fluoxetine; group 3 received mirtazapine plus veniafaxine;

and group 4 received mirtazapine plus bupropion. After one month, the score on the Hamilton Depression Rating Scale (HAM-D) was determined for each patient. The researchers compared the mean HAM-D scores of the four groups of depressed patients. For this study, identify each of the following:

a. experimental unit

b. response variable

c. factor

d. factor levels