| Sex, n(%) | |
| boys | 765(52.7) | 1463 (54.3) | 2228 (53.2) | 0.312 |
| girls | 687 (47.3) | 1230 (45.7) | 1917 (45.8) | |
| Age (years) mean (SD) a |
| 3–16 years | 8.46 (3.50) | 8.52 (3.66) | 8.50 (3.60) | 0.611 |
| 3–6 years | 5.04 (1.12) | 4.87 (1.17) | 4.93 (1.16) | |
| 7–9 years | 8.41 (0.84) | 8.36 (0.82) | 8.38 (0.82) | |
| 10–16 years | 12.70 (1.88) | 12.78 (1.86) | 12.75 (1.87) | |
| Place of residence, n(%) |
| ural | 1276 (86.7) | 1503 (55.3) | 2779 (66.4) | < 0.001 |
| urban | 195 (13.3) | 1213 (44.7) | 1408 (33.6) | |
| Only child, n(%) |
| yes | 309 (21.0) | 1274 (46.9) | 1583 (37.8) | < 0.001 |
| no | 1162 (79.0) | 1442 (53.1) | 2604 (62.2) | |
| Father’s educational level, n(%) |
| primary school or below | 185 (12.7) | 111 (4.2) | 296 (7.1) | < 0.001 |
| middle school | 833 (57.4) | 908 (34.1) | 1741 (41.6) | |
| high school | 303 (20.9) | 889 (33.4) | 1192 (28.5) | |
| junior college or bachelor | 125 (8.6) | 707 (26.5) | 832 (19.9) | |
| master or higher | 5 (0.3) | 49 (1.8) | 54 (1.3) | |
| Mother’s educational level, n(%) |
| primary school or below | 210 (14.8) | 193 (7.2) | 403 (9.6) | < 0.001 |
| middle school | 876 (61.7) | 966 (36.2) | 1842 (44.0) | |
| high school | 237 (16.7) | 847 (31.7) | 1084 (25.9) | |
| junior college or bachelor | 94 (6.6) | 635 (23.8) | 729 (17.4) | |
| master or higher | 3 (0.2) | 27 (1.0) | 30 (0.7) | |
| Marital status, n(%) |
| spinsterhood | 9 (0.6) | 17 (0.6) | 26 (0.6) | 0.081 |
| married | 1369 (95.1) | 2581 (96.6) | 3950 (94.3) | |
| divorced | 29 (2.0) | 40 (1.5) | 69 (1.6) | |
| widowed | 17 (1.2) | 16 (0.6) | 33 (0.8) | |
| remarried | 16 (1.1) | 17 (0.6) | 33 (0.8) | |
| Monthly income (RMB), n(%) |
| < 2000 | 375 (26.2) | 307 (11.5) | 682 (16.3) | < 0.001 |
| 2000–5000 | 698 (48.8) | 1131 (42.4) | 1829 (43.7) | |
| 5001–8000 | 160 (11.2) | 696 (26.1) | 856 (20.4) | |
| 8001–12,000 | 45 (3.1) | 219 (8.2) | 264 (6.3) | |
| > 12,000 | 17 (1.2) | 102 (3.8) | 119 (2.8) | |
| unknown | 134 (9.4) | 215 (8.1) | 349 (8.3) | |
