The study was approved by the Institutional Review Board of McGill University's Faculty of Medicine, Montréal. Parents of participants provided written informed consent, and participants provided assent. Data were drawn from the McGill University Study on the Natural History of Nicotine Dependence in Teens, a prospective investigation involving 1293 students, recruited from among all grade 7 classes in a convenience sample of 10 Montréal-area secondary schools.13
Baseline data were collected in self-report questionnaires in the fall of 1999, and follow-up data were collected at 3–4 month intervals over the next 5 years. Copies of the study questionnaire are available online in English (Appendix 1) and French (Appendix 2) at www.cmaj.ca/cgi/content/full/175/3/255/DC1
. About half (55.4%) of the eligible students participated at baseline. Nonparticipation was due to student reluctance to give a blood sample (for genotyping) and to a provincial labour dispute that resulted in some teachers refusing to collect consent forms. For our analysis, we included only those participants who reported that they had never smoked at baseline and who initiated cigarette smoking in any of the 19 follow-up survey cycles.
We used data on lifetime smoking to identify participants who reported at baseline that they had never smoked and who initiated cigarette smoking during follow-up. In each survey cycle, lifetime smoking was measured with the question “Have you ever in your life smoked a cigarette, even just a puff (drag, hit, haul)?” (No; Yes, 1 or 2 times; Yes, 3 or 4 times; Yes, 5 to 10 times; Yes, more than 10 times). Among 1293 students who participated in the baseline data collection, 814 had never smoked (not even a puff). Of these 814, 352 (43.2%) initiated smoking during follow-up (herein referred to as “initiators”), as measured by any affirmative response to the lifetime smoking question.
On the follow-up questionnaires, participants reported data on cigarette smoking during the 3 months preceding each questionnaire, including the number of days during each month that the subject had smoked and the number of cigarettes smoked per day on average during that month. Three-month test–retest reliability for these 2 items was good (kappa value 0.78 and 0.75, respectively).15
For 253 of the 352 initiators, time (day/month/year) of first puff was designated as the midpoint of the month during which the respondent first reported smoking cigarettes in the 3-month recall. For initiators who did not report smoking cigarettes in the 3-month recall, time of first puff was designated as the date of data collection in which lifetime use was first reported (n
= 25) or the midpoint of the year preceding the questionnaire in which they reported smoking in the past year in the self-perceived smoking status item (n
= 33). Self-perceived smoking status was measured with the item “Check the one box that describes you best” (I have never smoked a cigarette, even just a puff; I have smoked cigarettes, even just a puff, but not at all in the past 12 months; I smoked cigarettes once or a couple of times in the past 12 months; I smoke cigarettes once or a couple of times each month; I smoke cigarettes once or a couple of times each week; I smoke cigarettes every day). A total of 41 initiators were excluded from the analysis because time of first puff could not be determined, which left 311 initiators eligible for inclusion in this analysis ().
Fig. 1: Flow of participants through the study. NDIT = McGill University Study on the Natural History of Nicotine Dependence in Teens.
At each survey cycle following initiation, attainment of 12 events (milestones) related to either cigarette use (6 items) or to experiencing symptoms of nicotine dependence (6 items) was assessed. Of the 6 indicators of cigarette use, 5 indicators (first inhalation, monthly, weekly and daily smoking, and lifetime consumption of 100 cigarettes) have been used in surveys on smoking in youth16,17
or in studies on initiation of smoking in youth.11–14
Lifetime consumption of 100 cigarettes is commonly used to define the point at which smoking becomes established, and numerous studies only test cessation interventions among participants who have smoked at least 100 cigarettes in their lifetime.3,4
The 6 milestones describing symptoms of nicotine dependence were the most frequently reported symptoms among novice smokers in an earlier analysis of this database13
and are components of current diagnostic criteria for nicotine dependence in the DSM-IV18
The 6 milestones related to cigarette use included:
• Time of first inhalation: the survey date on which the participant first indicated ever taking cigarette smoke into his or her lungs for more than one puff
• Time of first whole cigarette: survey date on which the participant first reported ever smoking a whole cigarette (down to or close to the filter)
• Time of first monthly smoking: survey date on which the participant first responded “I smoke cigarettes once or a couple of times each month” in the self-perceived smoking status item
• Time of first weekly smoking: survey date on which the participant first responded “I smoke cigarettes once or a couple of times each week” in the self-perceived smoking status item
• Time of first daily smoking: survey date on which the participant first responded “I smoke every day” in the self-perceived smoking status item
• Time of first lifetime consumption of 100 cigarettes: survey date on which the participant first responded Yes to the question “Have you smoked 100 or more whole cigarettes in your life? (100 cigarettes = 4 packs of 25)”
The 6 milestones related to symptoms of nicotine dependence were as follows:
• Time of first self-report of physical addiction: survey date on which the participant first responded “a little,” “quite” or “very” to the question “How physically addicted to smoking cigarettes are you?” (Not at all; A little; Quite; Very)
• Time of first self-report of mental addiction: survey date on which the participant first responded “a little,” “quite” or “very” to the question “How mentally addicted to smoking cigarettes are you?” (Not at all; A little; Quite; Very)
• Time of first symptom of tolerance: survey date on which the participant first responded “a bit true” or “very true” to the statement “Compared to when I first started smoking, I can smoke much more now before I start to feel nauseated or ill” (Not at all true; A bit true; Very true)
• Time of first craving: survey date on which the participant first responded Yes to the question “Do you ever have cravings to smoke cigarettes? (No; Yes)
• Time of first withdrawal symptom: survey date on which the participant first responded “rarely,” “sometimes” or “often” to the question “Now think about the times when you have cut down or stopped using cigarettes or when you haven't been able to smoke for a long period (like most of the day). How often did you experience feeling a strong urge or need to smoke?” (Never; rarely; sometimes; often)
• Time of conversion to ICD-10 tobacco dependence
: survey date on which the participant first met 3 or more of the 6 ICD-10 criteria for tobacco dependence.19
The development, as well as the psychometric properties, of our measure of ICD-10 tobacco dependence for adolescents has been described previously.20
Data on the first 2 milestones of nicotine-dependence symptoms — mental and physical addiction — were collected in all survey cycles from all respondents whether or not they reported smoking cigarettes. These items were developed based on earlier qualitative work in which adolescents were asked to describe their experiences of nicotine dependence and were able to distinguish between what they perceived to be mental and physical addiction.21
Data on the remaining 4 milestones were collected in each survey cycle, but only among subjects who reported smoking cigarettes in the 3 months preceding the questionnaire. The test–retest reliability and the convergent construct validity of several of these indicators were tested in earlier work and are uniformly excellent.20
We described the natural course of onset of cigarette use among the 311 initiators in terms of the cumulative probability of having attained the milestone of interest according to time in months from smoking initiation (first puff). Probabilities were computed using Kaplan–Meier survival analysis, which is the method of choice when the length of follow-up differs between respondents and when censoring occurs (i.e., when a participant is lost to follow-up or reaches the end of the study without having experienced the milestone). The Kaplan–Meier curve is interpreted as the cumulative probability that the milestone of interest has occurred by a particular point.
We repeated this analysis for 3 subgroups of initiators: the subjects who had inhaled into their lungs (n = 253), the subjects who smoked a whole cigarette (n = 214) and the subjects who had experienced cravings (n = 141). We hypothesized that the temporal sequence of, and time interval between, milestones might differ in these subgroups.