regular smokers, measured either as the average or the maximum number of cigarettes per day (two measures highly correlated over time (>0.71) in longitudinally followed samples17). The mean quantity smoked per day varied between 13.1 and 23.4 cigarettes across the participating studies. Data on smoking cessation were available in N=35,845 individuals. Of these, 20,619 declared to have quit regular smoking (see above for the ‘ever regular smoker’ definition) at least 1 year prior to the time of the assessment, while the remaining participants were current smokers. Age at initiation, defined as when first smoked or age when started to smoke regularly (the genetic correlation between these two definitions exceeds .97, see 17) was observed in N=22,438 individuals. The average value as observed in the participating samples ranged from 16.4 to 32.2 years of age. Multidimensional scaling 18, STRUCTURE 19 or EigenSoft principal component analysis 20 were used to address population stratification prior to the study specific GWAS. Genomic control was further used to correct for potential effects of residual inflation 13. All the subjects from all studies participating in the TAG consortium provided informed consent.
