Sensitivity analysis
| Study and year of publication | Weight | SMD (95% CI) | I2 | p-value |
| Dubiley and Shogentsukova, 1973 [18] (three groups combined) | 12.53 | 0.35 (0.25–0.46) | 64 | <0.001 |
| Kolesar et al., 1977 [19] | 1.77 | 0.53 (0.43–0.63) | 85 | <0.001 |
| Boner et al., 1985 [20] | 1.41 | 0.52 (0.42–0.62) | 85 | <0.001 |
| Bobokhodzhaev and Shirinskii, 1990 [21] (two groups combined) | 4.75 | 0.51 (0.41–0.61) | 86 | <0.001 |
| Brimkulov, 1991 (2) [22] (two groups combined) | 14.19 | 0.48 (0.38–0.59) | 84 | <0.001 |
| Piacentini et al., 1993 [23] | 2.40 | 0.54 (0.44–0.64) | 85 | <0.001 |
| Simon et al., 1994 [24] | 1.59 | 0.53 (0.43–0.62) | 85 | <0.001 |
| Allegra et al., 1995 [11] | 1.32 | 0.53 (0.44–0.63) | 85 | <0.001 |
| Boner et al., 1995 [25] (two groups combined) | 2.23 | 0.53 (0.43–0.62) | 86 | <0.001 |
| van Velzen et al., 1996 [12] | 1.91 | 0.53 (0.44–0.63) | 85 | <0.001 |
| Kokov, 1996 [26] | 8.06 | 0.55 (0.45–0.65) | 85 | <0.001 |
| Valletta et al., 1997 [27] | 1.67 | 0.53 (0.44–0.63) | 85 | <0.001 |
| Grootendorst et al., 2001 [28] | 2.09 | 0.53 (0.43–0.63) | 85 | <0.001 |
| Peroni et al., 2002 [29] | 2.16 | 0.54 (0.44–0.64) | 85 | <0.001 |
| Straub et al., 2004 [30] | 5.76 | 0.55 (0.46–0.65) | 85 | <0.001 |
| Louie and Pare, 2004 [31] | 0.40 | 0.54 (0.44–0.63) | 85 | <0.001 |
| Peroni et al., 2009 [32] | 2.64 | 0.54 (0.44–0.64) | 85 | <0.001 |
| Huismans et al., 2010 [33] | 2.83 | 0.56 (0.46–0.65) | 84 | <0.001 |
| Rijssenbeek-Nouwens et al., 2012 [34] (two groups combined) | 16.32 | 0.58 (0.47–0.68) | 85 | <0.001 |
| Seys et al., 2013 [35] | 2.07 | 0.53 (0.43–0.62) | 85 | <0.001 |
| Verkleij et al., 2013 [14] (two groups combined) | 11.90 | 0.60 (0.49–0.70) | 85 | <0.001 |
Data are presented as % or n unless otherwise stated. SMD: standardised mean difference.