Sample size The expected sample size is calculated by the formula: n = [Z2 (1-α / 2) * p * (1-p)] / d2 = 36 patients n: minimum number of patients for the study; Z2 (1-α / 2): confidence factor at 95% probability level (= 1.96); p: the rate of patients with well recovering after surgery according to research by 7 Nguyen Trong Tin is 89.5%; d: permissible error = 0.1.