Daily smokers are individuals who smoked cigarettes 4 or more days per week. Age at onset for daily smoking was the AAO from the first interview in which the subject reported daily smoking. Habitual smokers are individuals who smoked at least one pack a day for 6 months or more. Non-habitual smokers are individuals who smoked more than 100 cigarettes lifetime but did not meet the criteria for habitual smoking. Sample characteristics are shown in Table 1.
