A hypothetical example illustrating that in data with 1000 cases and 1000 controls, if a SNP had altered frequency only in a subgroup of 200 cases, the OR would be skewed.

In the example, the allele frequency in the subclass (50%) masks the full effect of association, and moving the remaining cases to controls gives an OR of 2.08 for the SNP in the subclass. This corresponds to an OR error factor of 1.86, which can also be determined by inspecting the second lowermost curve in Figure 3 (case MAF 0.35) with a MAF increase in the subgroup of 0.15.