Table 3.

Selected main trial sample sizes (clusters per arm) accounting for the uncertainty in the ICC, for a range of cluster sizes, ICCs and pilot clusters per arm, calculated using result (6). Cluster size is assumed to be equal in pilot and main trials. Effect size d  =  0.25.

ICC	Clusters per arm	Swiger's method					Searle's method					Fisher's transformation
		Cluster size					Cluster size					Cluster size
		5	10	15	20	30	5	10	15	20	30	5	10	15	20	30
0.01	4	82	43	30	23	17	90	50	35	28	20	85	45	32	25	18
	8	78	41	28	22	16	82	44	31	24	17	80	42	29	23	16
	10	77	40	28	22	16	80	42	30	23	17	78	41	29	22	16
	15	76	40	27	21	15	77	41	28	22	16	77	40	28	22	16
	20	75	39	27	21	15	76	40	28	22	16	75	39	27	21	15
0.05	4	90	53	41	35	29	99	62	49	42	36	92	56	43	37	31
	8	86	51	39	34	28	90	55	43	37	31	87	52	41	35	29
	10	85	50	39	34	28	88	54	42	36	31	86	52	40	35	29
	15	84	50	39	33	28	86	52	41	35	30	84	51	40	34	29
	20	83	50	39	33	28	85	51	40	35	29	84	50	39	34	29
0.10	4	101	67	56	50	45	110	77	66	61	55	103	70	59	54	48
	8	97	65	55	50	44	102	70	60	54	49	99	67	57	51	46
	10	97	65	55	49	44	100	69	59	53	48	98	66	56	51	46
	15	96	65	54	49	44	98	67	57	52	47	97	66	56	50	45
	20	95	65	54	49	44	97	67	56	51	46	96	65	55	50	45
0.20	4	125	96	86	82	77	135	108	99	95	90	126	99	90	86	81
	8	122	95	86	81	77	128	101	92	88	83	124	97	88	84	79
	10	122	95	86	81	77	127	100	91	86	82	123	97	88	83	79
	15	122	95	86	81	77	125	98	89	85	80	123	96	87	83	78
	20	122	95	86	81	77	124	97	88	84	79	123	96	87	82	78

ICC: intra-cluster correlation.