The stepped wedge trial design: a systematic review

Table 3 Study size

Lead Author	Date	Number of 'steps'	Number of participants:		Time period between steps
			Intervention	Control
Gambia Hepatitis Study Group [4]	1987	17	61,065	63,512	10–12 weeks
Cook [17]	1996	2	371		Not stated
Wilmink [19]	1999	Individual – 13,147	29,713 person years	70,298 person years	Not stated – total 6 years
Somerville [14]	2002	2	119		1 year
Fairley [15]	2003	43	43		Not stated – total 20 weeks
Hughes [12]	2003	2	Aim: 304	Aim: 304	7 months (4 intervention, 3 wash-out)
Levy [16]	2004	Not stated (2 randomisation periods)	68		Not stated – total 20 weeks
Priestly [22]	2004	8	2,903	4,547	4 weeks
Bailey [18]	2004	4	400		About 3 months
Grant [20]	2005	1,655	1,655		Not stated – total 26 months
Ciliberto [21]	2005	7	992	186	3 weeks
Chaisson [13]	2005	29	Not stated		1 month

Notes:
The number of individuals is based on recruitment to the trial, rather than completed follow-up numbers.
Where only one figure for the number of participants is given, each individual/household participant receives the intervention at some stage during the trial.
In the Wilmink study [19], individuals cross-over from control to intervention at various points, but contribute person-years of data to both sections of the wedge.
In the remaining four studies with both 'intervention' and 'control' participants [4, 12, 21, 22], the unit of randomisation is the clinic or ward and hence an individual visiting the clinic/ward while it is in the control section of the wedge will not receive the intervention. Individuals visiting the clinic/ward once it has crossed-over to intervention will then contribute data to the intervention section of the wedge.

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