Fig. 4.

Fig. 4a (left) Subgroup analysis of effect size by study quality showed significant heterogeneity of effect size within the higher-quality group (I² = 80%, P=0.02) and the lower-quality group (I² = 99%, P<0.001) but no difference in subgroup means between the higher- and lower-quality groups. 4b (right) Subgroup analysis of effect size by publication year approached a significant difference in effect size between studies published prior to 2014 and those published in 2014 and later (I² = 99%, P = 0.06).