A case-only test for statistical interaction between each SNP and MHC risk score was carried out by a 1 df test based on the covariance between MHC risk score and the SNP genotype coded 0, 1 or 2. These tests were stratified within study by geographical region or by principal component score, and information pooled across strata and studies as described above. A 2 df test for association, possibly modified by MHC, was calculated by adding the chi-squared interaction test on 1 df to the 1 df chi-squared statistic for the stratified association test.
