To test the hypothesis that high-dose quetiapine (150-300 mg/day) monotherapy will be more effective for participants with melancholic depression the researchers will conduct a Cox regression analyses (Analysis C) comparing MADRS response rates between the quetiapine (150-300 mg/day) and placebo groups, while including the categorical melancholic depression grouping variable as a covariate and testing for an interaction between treatment group (quetiapine 150-300 mg/day vs placebo) and melancholic depression (present vs not present).