A square-root transformation was used for nicotine equivalents to achieve normal distribution. The sum of NNK metabolites was log-transformed. Least-square means for these two variables were computed for each genotype using the general linear model (GLM) procedure in SAS 9.0 (SAS, Inc. Cary, NC) adjusting for age, sex, race/ethnicity, cigarettes per day and body mass index, and further for nicotine equivalents. In the pooled-analysis, an adjustment variable for study was included. Tests of interaction were conducted between genotype and study to identify modifying effects, but were not statistically significant and, therefore, were not included in the final models. Tests for significance were two-tailed with an alpha level of 0.05.
