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Table 3 Study size

From: The stepped wedge trial design: a systematic review

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
  1. Notes:
  2. The number of individuals is based on recruitment to the trial, rather than completed follow-up numbers.
  3. Where only one figure for the number of participants is given, each individual/household participant receives the intervention at some stage during the trial.
  4. 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.
  5. 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.