Using the Q-Q plot, we examined the distribution of p values achieved for smoking status in our GWAS for all ∼380,000 SNPs that were analyzed (Appendix IV). As shown in the plot, the observed p values match well with the expected p values over a wide range of values of [−LOG10(p)], which is from 0 to ∼ 4. Observed p values gradually depart from expected p values at the extreme tail, where [−LOG10(p)] is ≥ ∼4. This pattern suggests that our GWAS association findings are more likely to be attributable to true genetic variation than to potential bias, such as genotyping errors.
