Table 2.

Methodological quality of studies included in the meta-analysis

First author (Publication year)	Student numbers	Randomization	Blind	Allocation concealment	Control for important factors	Control for incomplete data bias	Assessment of outcome	Total quality scores
Wu et al. (2015) [19]	1	1	0	1	2	1	2	8
Yuan (2014) [20]	3	0	0	1	2	1	2	9
Zhu (2014) [21]	3	0	0	1	2	1	2	9
Chao et al. (2013) [22]	1	0	0	1	2	1	2	7
Huang et al. (2013) [23]	2	0	0	1	2	1	2	8
Liu & Zhang (2013) [24]	1	0	0	1	1	1	2	6
Mi et al. (2013) [25]	1	0	0	1	2	1	2	7
Tao et al. (2013) [26]	2	0	0	1	2	1	2	8
Huang et al. (2012) [27]	1	0	0	1	1	1	2	6
Li (2012) [28]	3	1	0	1	2	1	2	10
Xu et al. (2012) [29]	2	0	0	0	2	1	2	7
Zhang et al. (2012) [30]	2	0	0	1	2	1	2	8
Wan (2011) [31]	3	0	0	1	2	1	2	9

Student numbers, a maximum of 3 scores could be awarded for this item. Studies in which student numbers were 50 to 81, 96 to 113, 118 to 270 received 1 score, 2 scores, 3 scores, respectively; Control for important factors, a maximum of 2 scores could be awarded for this item. Studies which were controlled for age received 1 score, and studies which were controlled for previous academic performance received an additional score;

Assessment of outcome, a maximum of 2 scores could be awarded for this item. Studies which were measured by examination scores received 1 score, and studies which were measured by both examination scores and questionnaire surveys received two scores