Table 1.

Comparison of the characteristics of students between retained samples and lost samples.

Control Variables	Retained Samples	Lost Samples	H0: (1) = (2) Difference
	Mean	Mean	Mean
	(SD)	(SD)	(p-Value)
	(1)	(2)	(3)
Students’ characteristics
(1) Age (1 = at least 10 years old; 0 = less than 10 years old)	0.83	0.87	0.04
	(0.37)	(0.34)	(0.35)
(2) Gender (1 = male; 0 = female)	0.51	0.56	0.05 ***
	(0.50)	(0.50)	(0.00)
(3) If the student is in grade-4 (1 = yes; 0 = no)	0.49	0.52	0.027 **
	(0.50)	(0.50)	(0.04)
(4) If the student has myopia (1 = yes; 0 = no)	0.16	0.12	−0.04 ***
	(0.37)	(0.33)	(0.00)
(5) Standardized mathematics scores at baseline	0.05	−0.14	−0.18 ***
	(0.98)	(1.02)	(0.00)
(6) Distance from the school to the student’s resident county (km)	34.31	34.11	−0.20
	(21.19)	(21.20)	(0.81)
Family characteristics
(7) ln (family assets)	9.66	9.60	−0.05 **
	(0.96)	(0.96)	(0.03)
(8) Father’s education level (1 = is at least a high school graduate; 0 = lower than high school)	0.13	0.13	−0.00
	(0.34)	(0.34)	(0.73)
(9) Mother’s education level (1 = is at least a high school graduate; 0 = lower than high school)	0.79	0.80	0.01
	(0.41)	(0.40)	(0.14)
(10) Both father and mother migrate to urban areas for work (1 = yes; 0 = no)	0.12	0.13	0.01
	(0.33)	(0.34)	(0.23)
N	16,685	3909

Note: *** p < 0.01, ** p < 0.05, * p < 0.1. SD, standard deviation. Data source: Authors’ survey.