The robustness of the statistical interaction was evaluated by varying the way in which CPD and age at onset were modeled. Finding an increased genetic association in early-onset smokers seems robust across CPD phenotypes and definitions of early onset. There was no statistical interaction observed between rs16969968 and sex, birth cohort, or educational attainment. The results of these analyses are detailed in eFigures 1 to 4 and eTables 1 and 2.
