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Table 2 Results from random-effect meta-analysis and meta-regression analysis

From: Placebo response and effect in randomized clinical trials: meta-research with focus on contextual effects

Study characteristic Trials (k) Patients (n) PCE (95% CI) τ2 I2 P
Overall (REML) 186 16,655 0.54 (0.46 to 0.64) 1.027 93.6  
Overall (REML, sensitivity analysis) 168 15,765 0.72 (0.67 to 0.79) 0.208 85.7  
Overall (D-L random) 186 16,655 0.59 (0.54 to 0.65) 0.253   
Overall (Fixed) 186 16,655 0.82 (0.80 to 0.83) 0.253   
Allocation concealment     1.005 93.6 0.024
 Clearly concealed 28 4322 0.82 (0.55 to 1.21)    
 Not clearly concealed 158 12,333 0.50 (0.41 to 0.59)    
Blinding of patients and providers     1.034 93.6 0.451
 Clearly a double-blind design 59 6477 0.62 (0.46 to 0.84)    
 Clearly not a double-blind design 97 8275 0.49 (0.39 to 0.62)    
 Unclear 30 1903 0.55 (0.37 to 0.82)    
Blinding of outcome assessor     0.967 93.5 0.002
 Clearly stated that outcome assessor was blinded 81 7614 0.72 (0.56 to 0.91)    
 Not stated that outcome assessor was blinded 105 9041 0.43 (0.34 to 0.54)    
Low risk of bias     1.016 93.6 0.084
 Clearly concealed allocation, dropout rate ≤15%, sample size > 49 16 3360 0.83 (0.50 to 1.40)    
 Criteria not fulfilled 170 13,295 0.51 (0.43 to 0.61)    
Information to participants     1.029 93.6 0.477
 Not informed that trial involved placebo 22 2150 0.64 (0.39 to 1.03)    
 Informed that trial involved placebo or not stated 164 14,505 0.53 (0.44 to 0.63)    
Time of outcome measurement     1.029 93.6 0.578
 < 4 weeks 83 6422 0.60 (0.47 to 0.76)    
 4–8 weeks 44 2614 0.55 (0.38 to 0.78)    
 > 8–12 weeks 27 2597 0.42 (0.27 to 0.65)    
 > 12 weeks 32 5022 0.50 (0.34 to 0.75)    
Type of intervention     1.005 93.4 0.026
 Pharmacological 56 6523 0.61 (0.45 to 0.82)    
 Physical 68 6649 0.64 (0.50 to 0.83)    
 Psychological 62 3483 0.38 (0.28 to 0.52)    
Type of outcome     1.029 93.7 0.523
 Patient-reported outcomes that are observable 42 3605 0.59 (0.41 to 0.84)    
 Patient-reported outcomes that are non-observable 88 7987 0.57 (0.45 to 0.72)    
 Observer-reported outcomes dependent on patient cooperation 25 1143 0.53 (0.34 to 0.83)    
Observer-reported outcomes that were not dependent on patient cooperation 22 1314 0.42 (0.24 to 0.74)    
 Laboratory outcomes 9 2606 0.31 (0.14 to 0.68)    
Settings     1.041 93.6 0.890
 Single center 97 5268 0.54 (0.42 to 0.68)    
 Multicenter 33 7394 0.51 (0.35 to 0.73)    
 Unclear 56 3993 0.57 (0.42 to 0.78)    
Patient´s condition     0.996 93.4 0.028
 Chronic condition 119 9771 0.47 (0.38 to 0.58)    
 Non-chronic condition 67 6884 0.68 (0.52 to 0.88)    
Type of outcome     1.015 93.6 0.083
 Binary outcome 39 5654 0.71 (0.50 to 1.01)    
 Continuous outcome 147 11,001 0.50 (0.42 to 0.60)    
Sample sizea     1.028 93.6 0.199
 ≤ 70 participants 93 2893 0.48 (0.37 to 0.61)    
 ≥ 71 participants 93 13,762 0.60 (0.48 to 0.74)    
Publication year     1.02 93.5 0.116
 Published before 2000 120 8961 0.49 (0.39 to 0.60)    
 Published in 2000 or later 66 7694 0.64 (0.49 to 0.83)    
Meta-regression of continuous variables Trials (k) Patients (n) Slope (95% CI) τ2 I2 P
Publication year 186 16,655 1.00 (0.98 to 1.02) 1.035 93.58 0.989
Sample size 186 16,655 1.00 (1.00 to 1.00) 1.026 93.54 0.162
Mean age 175 15,538 0.99 (0.98 to 0.99) 1.048 93.86 0.004
Percentage of females 163 15,259 1.01 (1.00 to 1.01) 0.8824 92.54 0.044
Placebo effect (SMD) 186 16,655 1.55 (1.07 to 2.24) 1.005 93.51 0.027
  1. k, number of trials; n, number of patients analyzed; τ2, estimate of between-study variance; I2, variation in PCE attributable to heterogeneity, estimated by random-effect subgroup analysis
  2. aSample size analyzed by dividing the trials in two groups, 70.5 (the median) being the cut-point