Final model of hierarchical regression for past 30-day smoking.

Hierarchical logistic regression was performed in boys and girls separately. All parental migration related variables were stepwisely selected using P<0.20 and P<0.25 as entry and removal criteria, whilst only type of parental migration met the criteria and entered in the model. Socio-demographic factors were adjusted among boys only, including school, grade, ethnicity, perceived living standard (as a surrogate of family income) and gender and education of the guardian.