Table 1.

Characteristics of the included studies. P-values are obtained from random-effect meta-regressions or simple regressions as described in the Supplementary Appendix, available as Supplementary data at IJE online (see Supplementary Appendix 1, available as Supplementary data at IJE online, for the definition of the covariates)

Trials	Studies with π = 0% (head-to-head trials) (n = 169, 100%)a	Studies with π between 20% and 50% (placebo-controlled trials)(n = 252, 100%)a	P-value
Median year of study completion [range]	1999	1999.5	0.71
	[1980, 2015]	[1978, 2014]
Use of rescue medication	66 (39.1%)	76 (30.2%)	0.06
Use of placebo run-in phase	71 (42.0%)	148 (58.7%)	0.001
No. of arms
2	158 (93.5%)	55 (21.8%)	<0.001
3	9 (5.3%)	142 (56.3%)
>3	2 (1.2%)	55 (21.8%)
Risk of bias
Generation of random sequence			0.41
Low risk of bias	30 (17.8%)	53 (21.0%)
Unclear risk of bias	139 (82.2%)	199 (79.0%)
Concealment of allocation			0.18
Low risk of bias	19 (11.2%)	40 (15.9%)
Unclear risk of bias	150 (88.8%)	212 (84.1%)
Blinding of assessors			0.04
Stated but not tested	34 (20.1%)	32 (12.7%)
Unclear risk of bias	135 (79.9%)	220 (87.3%)
Response rates available	154 (91.1%)	232 (92.1%)	0.73
Unpublished report available and presents adequate response data	50 (29.6%)	121 (48.0%)	<0.001
Response rate presented in published report	128 (83.1%)	163 (70.3%)	0.005
Published and unpublished reports on response are in agreement	139 (90.3%)	197 (84.9%)	0.126
All-cause dropout reported	153 (90.5%)	225 (89.3%)	0.68
Response and all-cause dropout reported	141 (83.4%)	213 (84.5%)	0.76
Dropout for adverse events reported	134 (79.3%)	220 (87.3%)	0.03
Funding			<0.001
High risk of sponsorship bias (Industry  funding or unclear)	159 (94.0%)	205 (81.3%)
Low risk of sponsorship bias (not-for profit  funding or no funding)	10 (6.0%)	47 (18.7%)

π: probability of receiving placebo.

^aPercentages are calculated out of the total number of trials reporting on response (154 and 232, respectively).