Table 1.

Characteristics of sample

Socio-demographic variables	n or mean	% or Standard deviation
Gender	5249
Boy	2703	51.5
Girl	2546	48.5
Grade
Grade one in junior middle school	806	15.3
Grade two in junior middle school	1221	23.3
Grade one in high middle school	1443	27.5
Grade two in high middle school	1010	19.2
Freshman	361	6.9
Sophomore	408	7.8
Location
Rural	2850	54.3
Urban	2399	45.7
Whether only child family
Yes	2468	47.0
No	2781	53.0
Age (years)
Boy	19.31	2.47
Girl	19.16	2.40