Figure 3.
Inverse variance weighted (IVW) estimator in the presence of heterogeneity. Ten single-nucleotide polymorphisms (SNPs) are simulated with true effects γj = 0.1, …1.0 on exposure X and true total effects Γj on outcome Y. Under the IVW model, each SNP yields a different ratio Γj/γj shown by the slopes of the dotted lines. The IVW estimate, shown by the slope of the solid line, is the mean of the ratios. Under homogeneity, all the dots would lie on the solid line. Heterogeneity creates uncertainty in the IVW estimate depending on the number of SNPs used as instruments.