The I2 statistic [25,26] describes the percentage of variation across studies that seems not to be due to chance. It is occasionally proposed as a measure of inconsistency, as opposed to heterogeneity, but the distinction of these terms is subtle and most authors still use them interchangeably. I2 is calculated as (Q-number of degrees of freedom)/Q. As shown by the formula, I2 differs from Q in that it also accounts for the number of studies. I2 has the advantage that it does not depend on the number of combined studies. It usually has large uncertainty and its 95% confidence intervals can be readily calculated [27].
