Although ∑rij2 is unknown in ICBP, a reasonable approximation is obtained by assuming that individually each SNP explains very little of the trait variance and thus ∑rij2 is constant and does not depend on j, i.e., ∑rij2=∑ri2, and solve for this quantity using the existing effect size estimate of βj from the available HapMap SNPs. Using ImpG assumes all HapMap SNPs were imputed without error; such error likely dampens the results.
