Table 3 Main variables measured in the X:IT study

Frequency of student smoking

Student self-reports

“How often do you smoke at present?”

Every day; not every day, but every week; not every week, but every month; more seldom than every month; never

Smokers are defined by ever smoking vs. never

Socioeconomic position

Student self-reports

“Does your father (mother) have at job?”

Yes; no; don’t know; don’t have/see my father (mother)

Information is coded into categories 1 (high) to 5 (low) according to the standards of the Danish National Research Institute of Social Research. Categories 6 (unclassifiable) and 7 (economically inactive) are added.

“Why doesn’t he (she) have a job?”

He (she) is: sick; a student; seeking work; retired; staying at home; don’t know

Family social class is coded at the highest ranking parent

Degree of implementation (Adherence, dose, quality of delivery, participant responsiveness)^a

School coordinator reports

(Adherence): “Are pupils allowed to smoke during school hours?”

Yes; yes outside school grounds; no

Adherence to the rules for smoke free school time are fulfilled if the school coordinator responds “no”

Student self-reports

(Dose): “How often do you see students smoke different places at school?”

Daily; sometimes; never

Dose are fulfilled if students report never seeing other students smoking at school