Table 1 Sample characteristic

Total

74,152 (100)

11,115 (100)

63,037 (100)

13,286 (100)

60,866 (100)

4047 (100)

70,105 (100)

Age, mean (SD)

14.7 (1.8)

14.6 (1.7)

14.7 (1.8)

< 0.001

14.5 (1.7)

14.7 (1.8)

< 0.001

14.3 (1.6)

14.7 (1.8)

< 0.001

Sex

 Male

36,895 (52.6)

4084 (39.1)

32,811 (54.9)

< 0.001

4655 (37.2)

32,240 (55.9)

< 0.001

1207 (32.3)

35,688 (53.8)

< 0.001

 Female

37,257 (47.4)

7031 (60.9)

30,226 (45.1)

8631 (62.8)

28,626 (44.1)

2840 (67.7)

34,417 (46.2)

Province

 Guangdong

10,159 (22.0)

1312 (19.3)

8847 (22.5)

< 0.001

1771 (21.9)

8388 (22.0)

< 0.001

398 (16.8)

9761 (22.3)

< 0.001

 Chongqing

9448 (6.1)

1569 (7.0)

7879 (6.0)

1980 (7.3)

7468 (5.9)

580 (7.4)

8868 (6.1)

 Shandong

8901 (18.1)

1164 (16.4)

7737 (18.4)

1289 (15.1)

7612 (18.8)

412 (16.3)

8489 (18.2)

 Liaoning

8977 (6.0)

1174 (5.5)

7803 (6.1)

1402 (5.4)

7575 (6.1)

426 (5.6)

8551 (6.0)

 Henan

9733 (23.6)

1493 (24.7)

8240 (23.4)

1791 (24.8)

7942 (23.4)

520 (24.8)

9213 (23.5)

 Yunnan

10,875 (10.1)

2011 (12.2)

8864 (9.7)

2076 (10.4)

8799 (10.0)

675 (11.4)

10,200 (10.0)

 Guizhou

8294 (9.6)

1379 (11.1)

6915 (9.4)

1678 (11.2)

6616 (9.3)

594 (13.0)

7700 (9.4)

 Heilongjiang

7765 (4.5)

1013 (3.8)

6752 (4.6)

1299 (4.0)

6466 (4.6)

442 (4.8)

7323 (4.4)

Residence

 Urban

34,971 (48.2)

5478 (50.7)

29,493 (47.8)

< 0.001

6684 (52.5)

28,287 (47.3)

< 0.001

2069 (53.4)

32,902 (47.9)

< 0.001

 Rural

39,181 (51.8)

5637 (49.3)

33,544 (52.2)

6602 (47.5)

32,579 (52.7)

1978 (46.6)

37,203 (52.1)

Having brothers/sisters

 Yes

53,955 (78.0)

8245 (79.0)

45,710 (77.9)

0.013

9857 (78.8)

44,098 (77.9)

0.032

3022 (79.3)

50,933 (78.0)

0.068

 No

20,197 (22.0)

2870 (21.0)

17,327 (22.1)

3429 (21.2)

16,768 (22.1)

1025 (20.7)

19,172 (22.0)

Living arrangement

 Living with parents

48,620 (66.8)

6557 (60.4)

42,063 (67.9)

< 0.001

7690 (59.4)

40,930 (68.4)

< 0.001

2246 (57.3)

46,374 (67.3)

< 0.001

 Living with a single parent

13,903 (18.3)

2515 (22.0)

11,388 (17.7)

3137 (23.0)

10,766 (17.3)

996 (23.9)

12,907 (18.0)

 Living with others

11,629 (14.8)

2043 (17.6)

9586 (14.4)

2459 (17.6)

9170 (14.3)

805 (18.8)

10,824 (14.6)

HSS

 Good

18,931 (27.8)

2394 (23.3)

16,537 (28.6)

< 0.001

2806 (23.1)

16,125 (28.8)

< 0.001

946 (25.1)

17,985 (27.9)

< 0.001

 Average

44,252 (59.4)

6571 (59.7)

37,681 (59.3)

7867 (59.6)

36,385 (59.3)

2258 (56.6)

41,994 (59.5)

 Poor

10,969 (12.9)

2150 (17.0)

8819 (12.1)

2613 (17.3)

8356 (11.9)

843 (18.2)

10,126 (12.6)

Father’s education level

 Primary school or below

44,050 (56.4)

6636 (56.8)

37,414 (56.3)

< 0.001

7906 (56.3)

36,144 (56.4)

0.106

2419 (56.3)

41,631 (56.4)

0.896

 Middle school

17,071 (24.3)

2397 (22.7)

14,674 (24.6)

2998 (23.7)

14,073 (24.5)

924 (24.7)

16,147 (24.3)

 University or above

13,031 (19.3)

2082 (20.6)

10,949 (19.1)

2382 (20.0)

10,649 (19.1)

704 (19.1)

12,327 (19.3)

Mother’s education level

 Primary school or below

46,811 (60.5)

6985 (60.0)

39,826 (60.6)

0.059

8373 (59.8)

38,438 (60.7)

0.008

2569 (60.5)

44,242 (60.5)

0.930

 Middle school

15,556 (22.0)

2276 (21.6)

13,280 (22.1)

2743 (21.6)

12,813 (22.1)

838 (21.8)

14,718 (22.0)

 University or above

11,785 (17.5)

1854 (18.4)

9931 (17.3)

2170 (18.6)

9615 (17.2)

640 (17.7)

11,145 (17.5)