Table 1 Demographic characteristics of study participants, n (%)
|  | BLBC n = 1026 | SLBC n = 1322 | DC n = 475 | NLBC n = 1160 | F or χ2 | P value |
|---|---|---|---|---|---|---|
Age, mean (SD) | 13.1 (1.3) | 13.1 (1.3) | 13.0 (1.3) | 12.8 (1.3) | 14.71 |  < 0.001 |
Gender | 4.00 | 0.261 | ||||
 Male | 569 (56.2) | 760 (58.3) | 248 (53.8) | 630 (55.1) |  |  |
 Female | 443 (43.8) | 543 (41.7) | 213 (46.2) | 513 (44.9) |  |  |
Grade | 17.30 | 0.001 | ||||
 Grade 5 Grade 6 | 449 (43.8) | 547 (41.4) | 219 (46.1) | 574 (49.5) |  |  |
 Grade 7 Grade 8 | 576 (56.2) | 774 (58.6) | 256 (53.9) | 585 (50.5) |  |  |
Income level | 72.10 |  < 0.001 | ||||
 Much better off/better off | 280 (27.6) | 315 (24.0) | 80 (17.0) | 341 (29.9) |  |  |
 The same | 670 (66.0) | 865 (66.0) | 315 (67.0) | 731 (64.1) |  |  |
 Poorer/much poorer | 65 (6.4) | 130 (9.9) | 75 (16.0) | 68 (6.0) |  |  |
Parental highest education level | 98.58 |  < 0.001 | ||||
 Primary school or lower | 126 (12.3) | 194 (14.7) | 65 (13.7) | 116 (10.0) |  |  |
 Middle school | 597 (58.2) | 723 (54.7) | 229 (48.2) | 519 (44.7) |  |  |
 High school or above | 190 (18.5) | 292 (22.1) | 119 (25.1) | 388 (33.4) |  |  |
 Do not know | 113 (11.0) | 113 (8.5) | 62 (13.1) | 137 (11.8) |  |  |
Only child | 114.14 |  < 0.001 | ||||
 No | 690 (67.3) | 936 (70.9) | 209 (44.1) | 764 (65.9) |  |  |
 Yes | 335 (32.7) | 385 (29.1) | 265 (55.9) | 395 (34.1) |  |  |
Household registration | 455.30 |  < 0.001 | ||||
 Rural | 699 (68.1) | 552 (41.8) | 232 (48.8) | 268 (23.1) |  |  |
 Urban | 327 (31.9) | 770 (58.2) | 243 (51.2) | 892 (76.9) |  |  |