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Table 2 Participant beliefs and voluntary run-in failure

From: Randomized controlled trials: who fails run-in?

  Full factorial randomization Two-group randomization
  Men (N = 1606) Women (N = 301) Women (N = 666)
  Randomized Voluntary run-in failure p Randomized Voluntary run-in failure p Randomized Voluntary run-in failure p
  N (%) N (%)   N (%) N (%)   N (%) N (%)  
  1423 (89) 183 (11)   252 (84) 49 (16)   584 (88) 82 (12)  
BASELINE FACTORS
 Preference: If you could choose, which kind of pill would you like to receive during the study?
  Calcium + vitamin D 854 (90) 96 (10) 0.08 174 (87) 27 (13) 0.10a 450 (88) 62 (12) 0.37
  Calcium only 67 (83) 14 (17)   13 (65) 7 (35)   34 (81) 8 (19)  
  Vitamin D only 63 (83) 13 (17)   8 (80) 2 (20)   N/A N/A  
  Placebo 32 (94) 2 (5.9)   4 (100) 0   N/A N/A  
  Don’t know/refused 407 (88) 57 (12)   53 (80) 13 (20)   99 (89) 12 (11)  
  Missing 0 1 (100)   0 0   1 (100) 0  
 Efficacy belief: How likely do you think it is that vitamin D supplements …
 Are helpful in preventing polyps    0.74    0.72    0.44
  Very/somewhat likely 852 (89) 108 (11)   173 (83) 36 (17)   381 (87) 58 (13)  
  Don’t know 530 (89) 67 (11)   70 (86) 11 (14)   182 (89) 23 (11)  
  Very/somewhat unlikely 40 (85) 7 (15)   8 (80) 2 (20)   20 (95) 1 (4.8)  
  Refused/missing 1 (50) 1 (50)   1 (100) 0   1 (100) 0  
 Improve health    0.20    0.42a    0.30
  Very/somewhat likely 1189 (89) 143 (11)   239 (84) 45 (16)   514 (87) 76 (13)  
  Don’t know 177 (85) 31 (15)   10 (77) 3 (23)   56 (90) 6 (9.7)  
  Very/somewhat unlikely 56 (88) 8 (13)   3 (75) 1 (25)   13 (100) 0  
  Refused/missing 1 (50) 1 (50)   0 0   1 (100) 0  
 Improve pain in bones and joints    0.59    0.63    0.18
  Very/somewhat likely 902 (89) 109 (11)   176 (83) 37 (17)   347 (86) 57 (14)  
  Don’t know 374 (88) 51 (12)   44 (85) 8 (15)   184 (91) 18 (8.9)  
  Very/somewhat unlikely 145 (87) 22 (13)   32 (89) 4 (11)   52 (88) 7 (12)  
  Refused/missing 2 (67) 1 (33)   0 0   1 (100) 0  
 Improve mood    0.31    0.47    0.06
  Very/somewhat likely 375 (87) 56 (13)   86 (83) 18 (17)   240 (84) 45 (16)  
  Don’t know 632 (90) 71 (10)   89 (87) 13 (13)   245 (91) 25 (9.3)  
  Very/somewhat unlikely 414 (88) 55 (12)   77 (81) 18 (19)   98 (89) 12 (11)  
  Refused/missing 2 (67) 1 (33)   0 0   1 (100) 0  
 Cause constipation    0.36    0.46    0.09
  Very/somewhat likely 240 (86) 38 (14)   82 (87) 12 (13)   117 (82) 25 (18)  
  Don’t know 669 (89) 85 (11)   91 (83) 18 (17)   282 (90) 33 (10)  
  Very/somewhat unlikely 511 (90) 59 (10)   79 (81) 19 (19)   184 (88) 24 (12)  
  Refused/missing 3 (75) 1 (25)   0 0   1 (100) 0  
 Efficacy scoreb mean (SD) compared with [randomized participants] 2.8 (2.6) 2.5 (2.9) 0.20 3.1 (2.5) 3.4 (2.7) 0.39 3.3 (2.6) 3.6 (2.9) 0.26
 Efficacy scoreb    0.16    0.94a    0.81
  −9 to −1 (benefit unlikely) 97 (85) 17 (15)   21 (88) 3 (13)   29 (88) 4 (12)  
  0 (no benefit) 149 (85) 26 (15)   16 (80) 4 (20)   49 (89) 6 (11)  
  1–5 (little benefit) 957 (90) 109 (10)   168 (84) 33 (16)   390 (88) 52 (12)  
  6–10 (some benefit) 220 (88) 31 (12)   47 (84) 9 (16)   116 (85) 20 (15)  
 Allocation belief: If you were to place a bet, which pill would you bet you’ll be given during the trial?
  Calcium + vitamin D 236 (90) 25 (9.6) 0.96 38 (83) 8 (17) 0.89a 116 (85) 21 (15) 0.73
  Calcium only 102 (89) 13 (11)   18 (82) 4 (18)   95 (88) 13 (12)  
  Vitamin D only 91 (90) 10 (9.9)   11 (79) 3 (21)   N/A N/A  
  Placebo 206 (89) 25 (11)   31 (79) 8 (21)   N/A N/A  
  Don’t know/refused 150 (91) 15 (9.1)   18 (90) 2 (10)   29 (88) 4 (12)  
  Missing/not askedc 638 (87) 95 (13)   136 (85) 24 (15)   344 (89) 44 (11)  
POST-ENROLLMENT FACTOR
 Perceived toxicity during the run-in periodd
  No 1412 (89) 169 (11) <0.0001a 249 (85) 43 (15) 0.001a 581 (89) 73 (11) <0.0001a
  Yes 11 (44) 14 (56)   3 (33) 6 (67)   3 (25) 9 (75)  
  1. Missing data are shown where values are not zero. p values are based on non-missing data
  2. aFisher exact test
  3. bEfficacy score was calculated as follows: For the first four health outcomes, extremely likely scores 2; somewhat likely scores 1; don’t know (or refused or missing) scores 0; somewhat unlikely scores –1; extremely unlikely scores –2. For constipation, the scoring system is reversed, e.g., very likely to cause constipation scores –2. All five scores are summed to give an overall efficacy score
  4. cThe number of respondents is smaller because this question was removed from the questionnaire
  5. dPerceived toxicities are defined as reports made by study coordinators on symptoms that participants attribute to the study tablets. These are identified either during the randomization questionnaire or if participants specifically contact study coordinators to report them