It is known from our prior work that the SNPs rs16969968, rs578776, rs13277254, rs12466358 are independently associated with nicotine dependence in the COGEND data [23]. The analyses for this study begins with multivariable logistic regression model including all four SNPs (additively coded as 0, 1, or 2 copies of the minor allele among those of European ancestry), number of H.S. peers who smoked (0–8) and the covariates gender and age. For each SNP, we tested for statistically significant multiplicative interaction between all model variables by adding a product term (e.g., rs16969968 × H.S. peer smoking) to the main effects model, retaining those that were statistically significant [28]. The Bonferroni corrected p-value used to test four hypothesized H.S. peer smoking by SNP interactions was 0.0125.
