Age at onset of regular smoking was dichotomized as 16 years and younger or older than 16 years, representing the median of smoking onset for smokers of all ages surveyed between 1992 and 2002 in the United States (http://riskfactor.cancer.gov/studies/tuscps/info.html). This definition corresponds to the dichotomization of age at onset of smoking used by Weiss and colleagues.10 Of the 94 050 current or former smokers in the samples, 61 128 had age at onset of smoking reported, and 33 348 of these were either heavy smokers (CPD >20) or light smokers (CPD ≤10). There were current and former smokers in both groups. These 33 348 individuals, analyzed separately at each site, represent the individuals included in the primary meta-analysis.
