AdoQuest is a prospective cohort investigation of grade 5 students aged 10–12 years at cohort inception designed to investigate the natural course of the co-occurrence of health-compromising behaviours in children. The sample was drawn from a stratified sample of schools selected from among all French-language schools with more than 90 grade 5 students, located in the greater Montreal area. To assure equal representation of students across socioeconomic status (SES), all schools located in the target territory were stratified using a continuous SES indicator  and 9–10 schools were randomly selected from within each of the upper, middle and lower SES tertile groupings. Participants were recruited from all grade 5 classes in each of the 29 participating schools. Participants provided written assent and parents/guardians provided written informed consent. The study received ethics approval from the Faculty of Medicine of McGill University, the Conseil sur l’éthique et la recherche, Concordia University and the Centre de Recherche du Centre Hospitalier de l’Université de Montréal.
Data for this cross-sectional analysis were collected in 2006–7 when students were aged 11–15 years and in their first year of secondary school (grade 7). Data on socio-demographic characteristics, depression, stress, and substance use were collected in classroom-administered or mailed self-report questionnaires completed by 1025 of the 1631 original participants (63%). The low response relates to challenges following up students from 29 elementary schools as they transitioned to more than 100 secondary schools. Parents completed a self-report questionnaire (either brought home by participants or mailed to participants’ homes, and then returned by mail), which collected data on parents’ socio-demographic characteristics, lifestyle habits and health.
The measure of lifetime stress or worry related to specific life events and relationships and incorporated cognitive appraisal of stress, according to Lazarus and Folkman’s Transactional Model of Stress . The measure reflects the tendency to worry or perceive an event as stressful. Data were collected in 12 items used previously in population-based surveys of Quebec adolescents . Participants reported if they had ever in their lifetime been stressed or worried (not at all, a little bit, quite a bit, a whole lot; not applicable) by any of: (1) your parents separating or divorcing; (2) breaking up with your boyfriend or girlfriend; your relationship with your (3) father; (4) mother; (5) brother(s)/sister(s); (6) friends; (7) a health problem (such as acne or asthma); (8) your weight; (9) sexual relations; (10) your new family (i.e., a reconstituted family); (11) financial problems in your family; and (12) school work. For analyses, responses were dichotomized into no (not at all, not applicable) or yes (a little bit, quite a bit, a whole lot), as in previous studies using this measure . It is well-established that divorce and blended families are associated with mental health symptoms and substance misuse in adolescence [41–46], and therefore the two items: “your parents separating or divorcing” and “your new family (i.e., a reconstituted family)” were combined into a single “family disruption” stress variable. Participants were categorized as having family disruption stress if their response was “yes” to either item. Due to accumulating evidence that stress related to romantic involvement during adolescence, and in particular during early adolescence, may also be associated with mental health symptoms, the item “breaking up with your boyfriend or girlfriend” was analyzed as a single item called “romantic breakup” stress [43, 47–49]. A principal axis factor analysis with direct oblimin (with delta = 0.0) as the method of extraction, was conducted with the remaining 9 items. The Kaiser-Guttman eigenvalue criterion and scree tests suggested a 2-factor solution. Two items (i.e., sexual relations stress, financial problems stress) did not relate strongly to either factor in the pattern matrix, and were dropped from further analyses. Based on these results and conceptual reasoning, Factor 1 had 4 items (your relationship with your father; mother; brother(s)/sister(s); friends) and was labelled "interpersonal” stress. Factor 2 had 3 items (health problem; weight; school work) and was labelled "personal” stress. Since Cronbach’s α for the personal stress factor was 0.55, we analyzed each item within the factor separately.
Depression symptoms were measured in a validated 6-item scale  which assessed how often (never, rarely, sometimes, often) in the past 7 days participants: (1) felt too tired to do things; (2) had trouble going to sleep or staying asleep; (3) felt unhappy, sad, or depressed; (4) felt hopeless about the future; (5) felt nervous or tense; (6) worried too much about things. Responses were summed and then divided by the number of items responded to, to create a depression symptom score which ranged from 1 to 4 (mean (sd) = 1.8 (0.7); median = 1.7, α = 0.82), with higher values indicating more frequent symptoms. For multivariable analysis, the score was dichotomized at 1 standard deviation above the mean of the sex-specific distributions (2.7 for girls; 2.3 for boys) , which resulted in 15.2% of participants being labeled as having depression symptoms. The remaining 84.8% were labeled as not having depression symptoms.
Presence or absence of conduct disorder symptoms was assessed in a 15-item screening scale based on the DSM-IV criteria for diagnosing conduct disorder . Participants reported any of the following (yes, no) in the past six months: (i) I have stayed out at night (until 4 or 5 am) despite my parents’ prohibitions; (ii) I have run away from home at least twice; (iii) I often bullied, threatened or intimidated others; (iv) I often initiated physical fights; (v) I have used a weapon (e.g., a knife, gun, chain, fist, bat, broken bottle) in a fight or to scare someone; (vi) I have been physically cruel to people; (vii) I have been physically cruel to animals; (viii) I have stolen while confronting a victim (e.g., mugging, purse snatching, extortion, armed robbery); (ix) I have forced someone into sexual activity; (x) I have deliberately engaged in fire setting with the intention of causing serious damage or to hurt someone; (xi) I have deliberately destroyed others’ property (objects, cars, buildings, broke windows); (xii) I have broken into someone else’s house, building, or car; (xiii) I often lied to obtain goods or favours or to avoid obligations; (xiv) I stole items of nontrivial value without confronting a victim (e.g., shoplifting; forgery); (xv) I was often truant from school. Participants were categorized as having conduct disorder symptoms if they responded yes to ≥3 of the 15 items. Cigarette smoking was assessed by asking participants if they had smoked a cigarette, even just a puff, in the past 6 months (yes, no). There is evidence of test-retest reliability of self-report smoking in youth , and there is strong agreement between self-reports and biochemical measures of tobacco use in prospective longitudinal studies, although recall beyond 1-year may be inconsistent [54, 55].
Questions used to measure alcohol, marijuana and illicit substance use were drawn from two population-based surveys in Canada [56, 57]. Binge drinking was assessed by asking participants to report the frequency of drinking ≥5 alcoholic drinks on the same occasion in the past 6 months [58, 59]. Responses (I have never drunk alcohol; I have never drunk ≥5 drinks on one occasion; I have drunk ≥5 drinks on one occasion, but not in the 6 months; 1–2 times; 3–5 times; 6–9 times; 10–19 times; 20–39 times; ≥40 times) were dichotomized into yes (any report of consuming ≥five alcoholic drinks on one occasion in the past 6 months) or no. Binge drinking dichotomized in this manner has been associated with risky health behaviours in youth [60, 61]. Test-retest reliability of self-report alcohol and other drug use measures, including marijuana, is high and use of short recall periods and simple language in stem questions leads to more accurate reporting .
Marijuana use was assessed by asking participants to report the frequency of consuming marijuana in the past 6 months. Responses (I have never consumed cannabis in my entire life; 1–2 times; 3–5 times; 6–9 times; 10–19 times; 20–39 times; ≥40 times; I don’t know what cannabis is) were dichotomized into yes (any marijuana use in the past 6 months) or no.
Use of illicit substances was measured by: “In your life, how often did you use the following drugs…(i) cocaine (coke, snow, crack, free base, powder); (ii) ecstasy (MDMA, E, X); (iii) hallucinogens (LSD, PCP, MESS, mushrooms, acid, mescaline, blotters); (iv) heroin (smack); (v) amphetamines (speed, upper); (vi) Ritalin (but NOT for medical reasons); (vii) tranquilizers taken without a prescription (downers, valium, Librium, dalmane, halcyon, ativan); (viii) steroids (testosterone, growth hormones, Dianobol, juice); (ix) Other drugs or medications taken without a prescription from a doctor?”. Response choices included: I have not used; just once to try; less than once a month (on occasion); about once a month; on the weekend OR 1–2 times a week; 3 or more times per week BUT not every day; every day. Because of the low endorsement of most items, we grouped all items into a single variable labelled “illicit drugs”. Participants were categorized as either yes (i.e., ever used one or more illicit drug) or no (i.e., never used any illicit drug).
Pubertal stage, assessed using the Pubertal Development Scale [62, 63], was classified as: (i) “pre-pubertal” if in girls, there was no body hair growth, no menstruation and no breast growth, or in boys, there was no body hair growth, no facial hair growth, and no deepening of the voice; (ii) “pubertal” if, in girls, there was any indication of body hair growth, breast growth or menstruation, or in boys, if there was any indication of body hair and/or facial hair growth and/or voice changes; or (iii) “post-pubertal” if in girls, body hair growth and breast growth were completed and menstruation was reported; or in boys, body and facial hair growth, and voice changes were completed.
Socio-demographic data included age and sex (from the student questionnaire), and mother completed university (yes, no), and annual household income (<$30,000CAN, $30,000-$99,999, ≥$100,000, missing) (from the parent questionnaire).