Sample size calculation

A non-inferiority design was chosen to evaluate whether CPOX with no preoperative PG is non-inferior to preoperative PG in bariatric patients. In patients with moderate or severe OSA, CPAP treatment is part of standard care. The primary outcome is QALY difference compared with costs, where QALYs are measured by the EQ-5D. Therefore, the sample size calculation is based on a predefined non-inferiority margin of 0.03 on the EQ-5D score. Based on an EQ-5D score of 0.68 in the usual care group, QALYs of patients with OSA before and after 1 year of CPAP treatment, and calculating with 80% power to detect the predefined non-inferiority margin at a one sided a level of 0.05, there are 621 patients needed in each study group.⁴⁴ Assuming a loss to follow-up of 10%, the total study population will be set at 1380 patients, resulting in 690 patients per arm.