Using the GWAS results, we obtain an estimate of h2 using LD score regression14 and an estimate of E(Rk2) from the predictive power of the GWAS-based polygenic score. Plugging these estimates into the above formula, we solve for an estimate of VGWAS. We then multiply this value by χGWAS2¯−1χMTAG2¯−1 (which we showed previously is equal to the ratio of the GWAS sample size to the MTAG’s GWAS-equivalent sample size) to obtain an estimate of VMTAG. Substituting this back into the above formula along with our estimate of h2 gives us the expected predictive power of the MTAG-based PGS.
