3.2 Employing the I2statistic
We investigated heterogeneity between studies by considering the
I2 method alongside the Chi2 P value. The I2 provides an estimate
of the percentage of inconsistency thought to be due to chance
(Higgins 2003). The importance of the observed value of I2 depends
on i. magnitude and direction of effects and ii. strength of
evidence for heterogeneity (e.g. P value from Chi2 test, or a confidence
interval for I2).We interpreted I2 estimate greater than or
equal to 50%accompanied by a statistically significant Chi2 statistic
as evidence of substantial levels of heterogeneity (Section 9.5.2
- Higgins 2008). When we found substantial levels of heterogeneity
in the primary outcome, we explored reasons for heterogeneity
(Subgroup analysis and investigation of heterogeneity).