Table 1.

Key characteristics of data sources

Country	England	Scotland
Data name	Smoking Drinking Drug Use (SDDU) survey	Scottish health survey
Years and interview dates	2012: September 2012 – December 2012 2014: September 2014 – December 2014 2016: September 2016 – January 2017	2012: January 2012 – December 2012 2014: January 2014 – February 2015 2016: January 2016 – January 2017
Individuals included	2012: 4915 2014: 3483 2016: 6920	2012: 319 2014: 271 2016: 232
Age range	11–15 years	13–17 years
Exposure to tobacco smoke in cars question	2012: Two separate questions: “In the past year, how often were you in your family’s car ( OR ) someone else’s car with somebody smoking?” (Responses: every day or most days; once or twice a week; once or twice a month; less often than once a month; never in the past year; don’t know) 2014 and 2016: In the past year, how often were you in a car with somebody smoking? This could be your family’s car or someone else’s car. (Responses: every day or most days; once or twice a week; once or twice a month; less often than once a month; never in the past year; don’t know) In this study ‘every day or most days’ classed as 'regular exposure'	2012–2016: “Are you regularly exposed to other people’s tobacco smoke in any of these places?” (responses: yes/no for in cars/vehicles, etc)
Deprivation marker	2012 and 2014: Child reported receipt of free school meals (FSM)	2012– 2016: Scottish index of multiple deprivation in five groups
	2016:Family Affluence Scale (low, middle, high). Harmonised so low groupe quivalent to receiving FSM	Harmonised so most deprived group comparable to receiving FSM or being in lowest affluence band