Table 1.

 Baseline characteristics of 3177 children with correctable refractive error by group assignment. Values are numbers (percentages) unless stated otherwise

Characteristics	Control group (n=1036)	Voucher group (n=988)	Free glasses group (n=1153)
Individual level
Mean (SD) age (years)	10.5 (1.1)	10.5 (1.1)	10.5 (1.1)
Male sex	517 (49.9)	474 (48.0)	563 (48.8)
Wearing glasses at baseline*	144 (13.9)	138 (14.0)	181 (15.7)
Gansu residence	397 (38.3)	356 (36.0)	405 (35.1)
Mean (SD) refractive error (diopters):
≤−2	410 (39.6)	374 (37.9)	431 (37.4)
≥−2 to −0.5	561 (54.2)	548 (55.5)	665 (57.7)
≤−0.5 to 0.5	18 (1.7)	17 (1.7)	10 (0.9)
≥0.5	47 (4.5)	49 (5.0)	47 (4.1)
Mean (SD) uncorrected visual acuity <6/18 in eye with better vision	602 (58.1)	525 (53.1)	686 (59.5)
Mean (SD) baseline standardized mathematics score	0.23 (1.01)	0.14 (0.98)	0.28 (0.98)
One or both parents with ≥12 years of education†	211 (20.4)	175 (17.8)	262 (22.8)
Family wealth‡:
Bottom third	333 (32.2)	351 (35.6)	368 (31.9)
Middle third	373 (36.1)	328 (33.2)	355 (30.8)
Top third	328 (31.7)	308 (31.2)	430 (37.3)
Boarding at school	235 (22.7)	183 (18.5)	286 (24.8)
Both parents out-migrated for work§	115 (11.1)	103 (10.4)	109 (9.5)
Blackboard use in class:
Little or none	277 (26.7)	260 (26.3)	456 (39.6)
Half of time	439 (42.4)	398 (40.3)	362 (31.4)
Most or all of time	320 (30.9)	330 (33.4)	335 (29.1)
Cluster level
Grade level (mean No/cluster (SD)):
Fourth grade	5.0 (3.4)	4.6 (3.7)	5.5 (4.8)
Fifth grade	7.3 (4.5)	7.3 (4.8)	8.3 (5.3)
Uncorrected visual acuity <6/18, in eye with better vision (mean No/cluster (SD))	7.2 (4.7)	6.3 (5.0)	8.2 (6.1)

*Defined as having glasses at school at baseline, having previously been told to bring them to school.

†Data missing for seven students.

‡Data missing for three students.

§Data missing for eight students.