Studies were divided among those dealing with samples with European ancestries, those with Asian ancestries, those with African ancestries, and those with Mexican (or Native American) ancestries. For studies that contained data from multiple populations, each was considered as effectively an independent study. Data from the studies were summarized by two-by-two tables. From each table a log-odds ratio and its sampling variance were calculated(Li et al. 2006). The Cochran’s χ2-based Q statistic test was computed in order to assess heterogeneity to ascertain whether each group of studies was suitable for meta-analysis. Where heterogeneity was found, the random effects model, which yields a wider CI, was adopted; otherwise, both the fixed and random effects models were adopted. A test for funnel plot asymmetry(Egger et al. 1997) was used to assess evidence for publication bias. The test used a linear regression approach to measure funnel plot asymmetry on the natural logarithm of the OR. The larger the deviation of each study from the funnel curve, the more pronounced the asymmetry. Results from small studies will scatter widely at the bottom of the
