The quantitative depressive symptom ratings were collected at Baseline, Day 1 (post ketamine), Day 3 using HDRS-24 (a 24-item questionnaire used to provide an indication of depression, and as a guide to evaluate recovery). The total score can range from 0 to a maximum score of 15 with a higher score indicating a worse outcome. A "responder" was defined as an individual exhibiting a reduction in the HDRS score from baseline to 24 hours (day 1) post-treatment, and all other individuals were classified as non-responders.