Table 2.

Sample size table for the CRISTAL trial*†

Event rate in experimental	Event rate in control	Overall event rate	Non-inferiority margin	N in each arm (individual)	Cluster size (for 31 clusters)	N total (cluster randomised)
Power=0.8
0.015	0.005	0.01	0.01	1553	56	3472
0.02	0.01	0.015	0.01	2319	88	5456
0.025	0.015	0.02	0.01	3076	123	7626
0.03	0.02	0.025	0.01	3826	163	10 106
0.035	0.025	0.03	0.01	4567	207	12 834
0.04	0.03	0.035	0.01	5301	258	15 996
0.0125	0.005	0.00875	0.0075	2420	92	5704
0.015	0.0075	0.01125	0.0075	3104	124	7688
0.0175	0.01	0.01375	0.0075	3784	160	9920
0.02	0.0125	0.01625	0.0075	4461	201	12 462
0.0225	0.015	0.01875	0.0075	5134	246	15 252
Power=0.9
0.015	0.005	0.01	0.01	2079	77	4774
0.02	0.01	0.015	0.01	3103	124	7688
0.025	0.015	0.02	0.01	4117	180	11 160
0.03	0.02	0.025	0.01	5121	245	15 190
0.015	0.0075	0.01125	0.0075	4154	182	11 284
0.0175	0.01	0.01375	0.0075	5065	241	14 942

*A one-sided α=0.025 is required for a 95% CI. The number of clusters is assumed to 31, the ICC=0.01 and the IPC=0.008.

†Table does not account for an estimation of loss to follow-up.

ICC, intracluster correlation coefficient; IPC, intermittent pneumatic compression.