Table 3.

Z-statistic boundaries and boundary crossing probabilities

Look	Information fraction	N	Cumulative alpha spent	Cumulative beta spent	Z-efficacy/harm	Z-futility	Boundary crossing probabilities under H1
							Efficacy	Futility
1	0.2	452	0.001	0.008	±3.252	±0.031	0.042	0.008
2	0.4	904	0.004	0.019	±2.986	±0.152	0.17	0.011
3	0.6	1355	0.009	0.036	±2.692	±0.631	0.281	0.017
4	0.8	1807	0.022	0.062	±2.374	±1.344	0.263	0.026
5	1.0	2259	0.05	0.1	±2.025	±2.025	0.143	0.038

Values were calculated using power = 0.90, alpha = 0.05, gamma spending function − 4 for the alpha and − 2 for the beta, expected incidence of postoperative pulmonary complications of 23% and 17.25% in the lower and higher positive end-expiratory pressure groups, respectively. Number of patients (N) is shown without correcting for dropouts. Look, interim analysis; H1, hypothesis 1 (group difference exists)