The primary purpose of this study is to assess whether at least one dose of LY2216684 (12 mg or 18 mg once daily) is superior to placebo once daily in the adjunctive treatment of patients with major depressive disorder (MDD) who were identified as partial responders to an adequate course of treatment with a selective serotonin reuptake inhibitor (SSRI), during an 11-week, double-blind, acute adjunctive treatment phase.