In both the NESDA and NTR samples, ND cases (N=611) were defined as scoring 4 or more on the Fagerstrom Test for Nicotine Dependence (FTND) (Heatherton et al., 1991) whereas ND controls (N=835) had FTND scores <4. For NTR subjects longitudinal FTND data (3 timepoints: 20003 timepoints: 2002 and 2004) were available in smokers and ex-smokers whereas for NESDA subjects FTND data were available in current smokers only. For the ex-smokers from NESDA (N=523) it was not possible to determine their FTND score for the period they smoked so they were excluded from analyses. For both samples the ‘never smokers’ (N=1208) were excluded because their liability to ND is unknown. Alcohol dependence was assessed by the DSM-IV based lifetime CIDI interview (version 2.1) in the NESDA sample (N=1760), with 318 AD cases and 1242 controls identified. The 200 participants identified as alcohol abusers were excluded. In the NTR sample (N=1777), participants were defined as being alcohol dependent if they ever reported a score of 2 or higher (N=237) on the CAGE (Ewing, 1984) questionnaire and classified as a control if they never scored 2 or higher (N=1526). Subjects with missing data on the CAGE were excluded (N=14).
