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Table 2 Covariate Involvement in the Design of Published RCTs a

From: Ideal vs. real: a systematic review on handling covariates in randomized controlled trials

  Covariates Involved in Allocation (N = 298)   Covariate-Adaptive Method (N = 237) b  
No Yes p-value No Yes p-value
N % N %   N % N %  
Type of Study      0.0140      0.4694
 Superiority 43 17 212 83   182 88 26 13  
 Non-Inferiority 5 22 18 78   14 78 4 22  
 Other 9 45 11 55   9 82 2 18  
Number of Arms      0.0246      0.7461
 2 40 17 196 83   167 87 26 13  
 3 5 21 19 79   16 84 3 16  
 4 7 27 19 73   15 83 3 17  
 5 or more 5 42 7 58   7 100 0 0  
Multicenter Study      0.0212      0.9843
 No 9 38 15 63   13 87 2 13  
 Yes 48 18 226 82   192 86 30 14  
Number of Centers, median (IQR) 12 (2–51) 24 (7–76) 0.8676 23 (7–76) 26 (6–64) 0.3399
Number of Subjects, median (IQR) 546 (200–1032) 584 (250–1442) 0.3601 580 (255–1369) 613 (188–1389) 0.5002
Length of Study, Years, median (IQR) 3 (2–4) 3 (2–5) 0.0158 3 (2–5) 3 (3–7) 0.0403
Months of Subject Follow-up, median (IQR) 6 (3–18) 12 (6–22) 0.1825 12 (6–19) 12 (6–36) 0.5449
Study Initiation Year      0.0889      0.0066
 Unable to Determine 1 20 4 80   3 75 1 25  
 Before 2000 1 6 16 94   11 69 5 31  
 2000–2004 13 17 62 83   49 82 11 18  
 2005–2009 23 19 100 81   87 89 11 11  
 2010 and later 19 24 59 76   55 93 4 7  
Primary Outcome      0.0054      0.4861
 Continuous 23 26 66 74   55 86 9 14  
 Binary 19 17 94 83   82 89 10 11  
 Time-to-Event 9 11 75 89   64 85 11 15  
 Other 6 50 6 50   4 67 2 33  
Number of Covariates (involved in randomization algorithm)           <.0001
 1 Not applicable for this outcome.   93 98 2 2  
 2   78 93 6 7  
 3   30 77 9 23  
 4   4 40 6 60  
 5 or more   0 0 9 100  
  1. aP-values correspond to logistic regression results
  2. bThis subset of 237 studies includes those studies clearly involving either stratified block method (N = 205) or an adaptive method (N = 32). Refer to Table 1 (205 + 32 studies using stratified block method or covariate-adaptive method, respectively)
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