The between-study variance, τ2, is the most direct measure of the magnitude of heterogeneity, but is rarely reported. When seen in isolation, it is difficult to interpret and it cannot be compared across different meta-analyses. If we want to see the magnitude of the heterogeneity juxtaposed to the magnitude of the effect size, a more useful metric may be h, which is defined as the ratio of τ over the effect size, i.e. it says how big the between-study standard deviation is compared with the summary effect size. A large h may be seen with large between-study variance or a small effect size [28].
