Statistical analysis
The statistical analyses were performed using R version 2.14 [16-17]. The meta pack was used to calculate and generate the pooled estimates and forest plots. Publication bias was also investigated statistically using Egger and Begg tests [18-19]. Statistical heterogeneity was assessed using the I2 test, with a value ≥ 50% indicating a substantial level of heterogeneity. Random effects models would be used when substantial statistical heterogeneity was present. A p-value ≤ 0.05 was defined as statistically significant.