We used Stata version 10 with the metan command. We judged a p value less than 0∙05
significant. We used a random-effects model in all analyses because we expected
the data to be heterogeneous across studies. We calculated I² values to test for heterogeneity between studies. We deemed I² less than 25% to have low heterogeneity, 25–75% to have medium heterogeneity, and greater than 75% to have high heterogeneity. We also assessed
publication bias and selective reporting with Egger’s and Begg’s tests, and by inspecting the symmetry of funnel plots, as recommended in the Cochrane handbook.