Assessment of heterogeneity
We used the Chi*2 test on N-1 degrees of freedom with a significance level of P less than 0.05, and the IQ statistic to examine the heterogeneity among trials. A guide to interpretation is as follows, as described in the Cochrane Handbook (Higgins 2011) (16). IQ statistic values of 25%, 50% and 75% correspond to low, moderate and high levels of heterogeneity, respectively. If the IQ statistic estimate is greater than 50%, we used a random effect model to account for such heterogeneity.