Similar to the reasoning with the genotype data, the studies were once again reanalyzed following stratification by majority ethnicity, either Asian or Caucasian (Figure 6). This, however, did not reduce the heterogeneity in either group to acceptable levels: for Asian studies the observed I-squared = 90.1%, χ2 (4) = 40.25, P < 0.0001; and for Caucasian studies the observed I-squared = 45.0%, χ2 (8) = 14.54, P = 0.07. Therefore, in this instance, stratifying by majority ethnicity was not useful in accounting for the high degree of heterogeneity between studies. Furthermore, grouping the studies with regard to Caucasian and Asian ethnicity did not alter the association between allele frequencies and opioid dependence, with a pooled OR (95% CI) for Caucasian studies of 1.13 (0.90–1.42), P = 0.29, and for Asian studies of 1.31 (0.77–2.22), P = 0.33.
