The analysis sample contains 797 cases and 811 controls of European descent. The average age at interview for nicotine dependent cases was 35.7 years (SD=5.5), the average age for controls was 36.9 years (SD=5.3); this was a small, but significant difference (p<0.001). There were significantly more women among controls than cases (70% vs. 56%, p<0.001). As such, age and sex were included as covariates in all analyses. To test for the possibility of population sub-structure not accounted for by self-reported race/ethnicity, STRUCTURE analyses (Pritchard et al. 2000) were conducted; no evidence of confounding substructure was detected (Bierut et al. 2007). This was subsequently confirmed using principal components analysis in Eigenstrat (Li and Yu 2008). Mean AOS was 15.4 years for cases (SD=3.6) and 18.0 years for controls (SD=3.6); a highly significant difference (ANOVA t=13.55, p<0.001).
