Figure 2.
Simulation results showing bias in estimates of the maternal and fetal effect of the human leukocyte antigens X1 allele obtained using structural equation modelling (upper row) or ordinary least squares linear regression (lower row). The true fetal effect of human leukocyte antigen's (HLA)-X1 varies between the six panels (βfX1 = 0 left panels; βfX1 = 0.014 middle panels; βfX1 = 0.02 right panels—the fetal effects of other HLA-X alleles for each panel are listed in Supplementary Table S1, available as Supplementary data at IJE online). The effect of the maternal alleles (βmX1, βmX2, βmX3) varies across the x-axis. Across all conditions, the structural equation model returned unbiased estimates of the maternal and fetal effect size for HLA-X1 whereas ordinary least squares regression resulted in biased effect estimates. Similar patterns of results were observed for HLA-X alleles other than HLA-X1 (results not shown)