Sample size calculation for a stepped wedge trial

Table 1 Estimated number of clusters for three sample size calculation methods used in SWTs, as a function of the ICC and outcome type (continuous, binary and rate) to obtain 80 % power

ICC	Standard CRT inflation factor		DE inflationfactor based onWoertman et al.	Analytical power based on HH
	K=20,J=1	K=120,J=1	K=20,J=6	K=20,J=6
Continuous outcome^a
0	26	5	8	9
0.1	74	55	12	12
0.2	122	105	11	11
0.3	170	155	10	10
0.4	218	205	9	9
0.5	266	256	7	7
Binary outcome^b
0	25	5	8	10
0.1	71	53	12	13
0.2	117	101	11	12
0.3	163	149	9	11
0.4	209	197	8	10
0.5	256	246	7	8
Count outcome^c
0	24	4	8	8
0.1	69	51	11	11
0.2	114	98	10	10
0.3	159	145	9	9
0.4	203	192	8	8
0.5	248	238	7

^aIntervention effect = −0.3785; σ _y=1.55; sample size for a parallel RCT = 253 subjects per arm.
^bBaseline outcome probability = 0.26; OR = 0.56; sample size for a parallel RCT = 243 subjects per arm.
^cBaseline outcome rate = 1.5; RR = 0.8; sample size for a parallel RCT = 236 subjects per arm.
Notation: K= number of subjects per cluster; J= total number of time points, including one baseline

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