The 2 samples used in this study were selected using different criteria. In the VAND sample, the 3 groups were never smokers, low-ND smokers (FTQ score ≤2), and high-ND smokers (FTQ score ≥7). The VAANX sample is a 2-group, case-control design for studying neuroticism and anxiety.56 Because the selection was based on a composite genetic factor score, we do not have a sufficient number of participants for a dichotomized design for studying ND. The best comparable phenotype in this sample to ND is the FTND score. For this reason, we used a quantitative design to approximate the ND phenotype in the VAND sample. As reported in the literature,45,65,66 the FTND is a reliable instrument to assess ND. However, our dichotomized ND definition in the VAND sample is not the same as the FTND scores in the VAANX sample. This is a potential weakness of our study that may contribute to the difference in association signals observed between the 2 samples.
