 Study sample size was calculated based on actual or assumed remission rates, the primary outcome parameter of the study. A remission rate of 60 was set for the reference treatment ECT. A non-inferiority limit of 40 was set for the experimental intervention ketamine. This is an arbitrary level, based on an assumption of fewer and milder side effects, faster antidepressant effect and the fact that the patients do not need to be anaesthetized and given muscle relaxants. With the above limits, a power of 80 and a significance level of 5, 97 patients are required in each arm, according to: n 2 8,4 p1-p difference2, where 2 and 8,4 are derived from significance and power levels, p are the actual and assumed levels for the proportion of patients reaching remission for ECT and ketamine 60 and 40 respectively. The size of the cohort is calculated to be sufficiently large to detect ECT-associated cognitive side effects. 