smoking duration among current smokers. Individuals who indicated having smoked fewer than 100 cigarettes in their lifetime were classified as having an average lifetime cigarette use of zero pack-years. The distribution of pack-year values was skewed, so the data were log10 transformed prior to data analysis to approximate a normal distribution. Pack-years was strongly associated with a prior measure of the number of lifetime nicotine dependence symptoms (r = 0.70, p < 0.0001) among a sub-sample of individuals who participated in a study of drug dependence in 1992 (Xian et al. 2007).
