APPENDIX 2.

Covariates and their assessment

Variable	Survey question	Response categories
Socio-demographics
Age	How old are you?	Years
Sex	Are you a girl or a boy?	Boy/girl
Family Affluence Scale (Cronbach α=0.44)	Does your family own a car, van or truck?	No/yes, one/yes, two or more
	Do you have your own bedroom for yourself?	No/yes
	During the past 12 months, how many times did you travel away on holiday with your family?	Not at all/once/twice/more than twice
	How many computers does your family own?	None/one/two/more than two
Personal characteristics
School performance	How would you describe your grades last year?	Excellent/good/average/below average
TV screen time	On a school day, how many hours a day do you usually spend watching TV?	None/<1 h/1–2 h/3–4 h/more than 4 h
Number of movies seen	Below is a list of film titles. Please mark if, and how often, you have seen each film.	Never/once/twice/more than twice
Sensation seeking/rebelliousness (Cronbach α=0.70)
	How often do you do dangerous things for fun?	Not at all/once in a while/sometimes/often/very often
	How often do you do exciting things, even if they are dangerous?	Not at all/once in a while/sometimes/often/very often
	I believe in following rules (recoded).	Not at all/a bit/quite well/very well
	I get angry when anybody tells me what to do.	Not at all/a bit/quite well/very well
Social environment
Peer smoking	How many of your friends smoke cigarettes?	None/a few/some/most/all
Mother smoking	Does your mother/female guardian smoke cigarettes?	Yes/no/don't know (coded `no')/don't have (coded `no')
Father smoking	Does your father/male guardian smoke cigarettes?	Yes/no/don't know (coded `no')/don't have (coded `no')
Sibling smoking	Do any of your brothers or sisters smoke cigarettes?	Yes/No/don't have (coded `no')