Table 1 Descriptive characteristics of the study population

Sex, n (%)

 Boys

1557 (53.5)

631 (58.9)

926 (50.3)

 < 0.001

 Girls

1356 (46.5)

440 (41.1)

916 (49.7)

Grade, n (%)

 Lower grades (Grade 1 to 3)

1489 (51.1)

530 (49.5)

959 (52.1)

0.180

 Upper grades (Grade 4 to 6)

1424 (48.9)

541 (50.5)

883 (47.9)

 BMI

16.72 (15.22, 18.92)

17.13 (15.38, 19.18)

16.49 (15.14, 18.74)

 < 0.001

 Height

139.00 (130.00, 149.35)

140.00 (130.50,150.00)

138,50 (130.00,149.00)

0.018

Parental myopia, n (%)

 None

681 (23.4)

147 (14.5)

534 (31.0)

 < 0.001

 One

1023(35.1)

392 (38.7)

631 (36.6)

 Both

1034 (35.5)

474 (46.2)

560 (31.9)

 Unclear

175 (6.0)

58 (5.4)

117 (6.4)

Maternal education level, n (%)

  ≤ High school

987 (33.9)

205 (19.2)

782 (42.5)

 < 0.001

  ≥ Undergraduate

1926 (66.1)

868 (80.8)

1058 (57.5)

Paternal education level, n (%)

  ≤ High school

1162 (39.9)

298 (27.6)

864 (47.0)

 < 0.001

  ≥ Undergraduate

1751 (60.1)

776 (72.4)

975 (53.0)

Close-work activities, n (%)

 Homework on weekdays > 1 h

2423 (84.5)

805 (86.3)

1518 (83.5)

0.045

 Homework on weekends > 1 h

2414 (85.1)

907 (87.2)

1507 (83.9)

0.017

 Cell phone use on weekdays > 1 h

817 (28.7)

278 (26.7)

539 (29.8)

0.075

 Cell phone use on weekends > 1 h

1232 (43.9)

450 (43.8)

782 (44)

0.922

 Computer use on weekdays > 1 h

369 (13)

135 (13)

234 (12.9)

0.965

 Computer use on weekends > 1 h

536 (19.1)

188 (18.4)

348 (19.5)

0.461

 Outdoor activities on weekdays > 2 h

974 (34.4)

376 (36.4)

598 (33.2)

0.088

 Outdoor activities on weekends > 2 h

1528 (54.4)

574 (55.9)

954 (53.5)

0.211

 Sleep every night > 9 h

368 (13.1)

118 (11.4)

250 (14)

0.046

 Attend learning classes ≥ 3

859 (30.3)

347 (33.3)

512 (28.5)

0.008