Where N is the combined sample size, Z¯is and σZis2 are the sample mean and variance of Z in the ith genotype group of the sth study, respectively. When combining summary-level data to calculate the Levene’s test statistics L, the following natural weights ωis and γi were calculated: ωis=nis∑snis and γi=niN, where ni the sum of genotype counts in the ith genotype group across all participating cohorts. These weights are determined by the frequency of the marker amongst the cohorts, such that the sum of both weights is equal to 1, i.e. ∑sωis=1 and ∑iγi=1. The meta-analysis Levene’s test P-value is obtained by comparing L to an F-distribution with df1 = 2 and df2 = N-3.