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Table 1 Study Characteristics a

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

Study Characteristic   Overall 2009 2014 p-value b
Type of Study Superiority 255 (86) 110 (84) 145 (87) 0.3047
Non-Inferiority 23 (8) 9 (7) 14 (8)  
Other 20 (7) 12 (9) 8 (5)  
Cluster-Randomized No 277 (93) 117 (89) 160 (96) 0.0297
Yes 21 (7) 14 (11) 7 (4)  
Number of Arms 2 236 (79) 107 (82) 129 (77) 0.5970
3 24 (8) 8 (6) 16 (10)  
4 26 (9) 12 (9) 14 (8)  
5 or more 12 (4) 4 (3) 8 (5)  
Multicenter Study Single Center 24 (8) 15 (11) 9 (5) 0.0563
Multicenter 274 (92) 116 (89) 158 (95)  
Number of Centers   22 (5–71) 20 (4–68) 24 (6–76) 0.6291
Number of Subjects   556 (242–1408) 700 (279–1888) 457 (222–1224) 0.0770
Length of Study, Years (if reported)   3 (2–5) 3 (2–5) 3 (2–5) 0.6667
Months of Subject Follow-up (if reported)   12 (4–20) 12 (4–22) 12 (4–19) 0.8274
Study Initiation Year Before 2000 17 (6) 13 (10) 4 (2) <.0001
2004–2005 75 (26) 66 (52) 9 (5)  
2005–2009 123 (42) 48 (38) 75 (45)  
2010 and later 78 (27) 0 (0) 78 (47)  
Primary Outcome Continuous 89 (30) 42 (32) 47 (28) 0.2663
Binary 113 (38) 42 (32) 71 (43)  
Time-to-Event 84 (28) 40 (31) 44 (26)  
Other 12 (4) 7 (5) 5 (3)  
Randomization Indicated in Title No 94 (32) 40 (31) 54 (32) 0.7398
Yes 204 (68) 91 (69) 113 (68)  
Allocation Method Purely random allocation 4 (1) 3 (2) 1 (1) 0.1660
Blocked (Permuted or Random block) 24 (8) 9 (7) 15 (9)  
Stratified or Stratified block 205 (69) 82 (63) 123 (74)  
Minimization/Covariate-adaptive method 32 (11) 18 (14) 14 (8)  
Other 4 (1) 2 (2) 2 (1)  
Unable to determine 29 (10) 17 (13) 12 (7)  
Covariate(s) Included in Allocation No 27 (9) 12 (9) 15 (9) 0.3278
Yes 241 (81) 102 (78) 139 (83)  
Unable to determine 30 (10) 17 (13) 13 (8)  
Number of Covariates 1 95 (39) 42 (41) 53 (38) 0.5106
2 86 (36) 32 (31) 54 (39)  
3 40 (17) 17 (17) 23 (17)  
4 11 (5) 5 (5) 6 (4)  
5 or more 9 (4) 6 (6) 3 (2)  
Baseline Test for Significant Differences No 167 (57) 76 (59) 91 (55) 0.5564
Yes 126 (43) 53 (41) 73 (45)  
Unadjusted or Adjusted Analyses Unadjusted 49 (16) 27 (21) 22 (13) 0.0100
Adjusted 87 (29) 27 (21) 60 (36)  
Both 162 (54) 77 (59) 85 (51)  
Reason for Adjustment (if applicable) Pre-specified 226 (91) 88 (85) 138 (95) 0.0778c
Data driven - Lack of balance 5 (2) 3 (3) 2 (1)  
Data driven - Potential confounding 6 (2) 4 (4) 2 (1)  
Other 4 (2) 3 (3) 1 (1)  
Unable to determine 8 (3) 6 (6) 2 (1)  
  1. aPercentages were calculated based on non-missing values
  2. bP-value corresponds to chi-squared test result or Wilcoxon Rank-Sum test result for comparison of study characteristics by publication year, as appropriate
  3. cWithin the subset of trials reporting adjusted analyses, 95% of trials from 2014 vs. 85% of those from 2009 pre-specified adjustments (p = 0.0045)
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