Table 4. Final model of hierarchical regression for past 30-day smoking.

	ORadj.	95% CI	P value
Boys
Type of parental migration
No parental migrating (reference)	1.00
Father migrating only	0.54	(0.29, 1.00)	0.050
Mother migrating only	1.91	(0.81, 4.51)	0.140
Both migrating	0.86	(0.43,1.73)	0.671
Self-efficacy
Middle+high (reference)	1.00
Low (<25%)	5.78	(3.53, 9.48)	<0.001
Girls
Type of parental migration
No parental migrating (reference)	1.00
Father migrating only	0.87	(0.16, 4.79)	0.870
Mother migrating only	4.50	(0.46, 43.66)	0.194
Both migrating	1.15	(0.12, 10.76)	0.900
Self-efficacy
Middle+high (reference)	1.00
Low (<25%)	4.75	(1.10, 20.52)	0.037

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.