It often takes several weeks to find out if an antidepressant medication is going to work for someone. This research study aims to identify tests that are able to predict if a medication will work, even before a person starts to feel better. The first test is a measurement of the blood protein Brain-Derived Neurotrophic Factor (BDNF), which is involved with brain cell growth. The second test is a Quantitative Electroencephalogram (QEEG), which measures brain activity.