We used the FTND range of scores to categorize participants' nicotine dependence as mild (FTND score 0–3), moderate (FTND score 4–6) or severe (FTND score 7–10). In the deCODE Genetics sample only, 4313 low-intensity smokers (10 or fewer CPD) with no FTND data available were added into the mild category. Among participants with both CPD and FTND data available in two samples (deCODE and COGEND), we found high concordance rates (~90%) between a report of 10 or fewer CPD and FTND scores of 3 or less, meaning that the inclusion of low-intensity smokers would enable us to increase sample size with little phenotype misclassification. As presented in Table 1, our final analysis data sets totaled 17 074 participants of European-ancestry: 9137 with mild, 4881 with moderate and 3056 with severe nicotine dependence.
