Table 1
Characteristics of study participants by maternal education level (%)

Characteristics		Total	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019
		(595,327)	(61,347)	(59,611)	(63,129)	(61,357)	(58,475)	(57,904)	(54,219)	(52,357)	(49,441)	(47,445)	(30,042)
Sex	Boy	50.6	51.2	51.4	51.2	51.2	50.9	50.5	50.6	50.6	50.6	50.5	44.9
	Girl	49.4	48.8	48.6	48.8	48.8	49.1	49.5	49.4	49.4	49.4	49.5	55.1
School	Middle school	45.5	47.8	47.2	46.8	46.1	45.2	45.2	44.1	42.5	42.6	43.7	49.1
	High school	54.5	52.2	52.8	53.2	53.9	54.8	54.8	55.9	57.5	57.4	56.3	50.9
Grade	Middle 1st	13.7	14.8	14.5	14.2	13.8	14.0	13.0	12.1	12.9	13.3	12.8	16.4
	Middle 2nd	15.2	16.1	15.7	15.8	15.3	14.9	15.8	14.6	13.5	14.5	14.9	15.4
	Middle 3rd	16.5	16.9	16.9	16.8	17.0	16.3	16.4	17.4	16.1	14.8	16.0	17.3
	High 1st	17.8	17.6	17.5	17.9	17.8	18.0	17.7	18.0	19.1	17.7	16.5	17.2
	High 2nd	18.1	17.5	17.5	17.6	18.1	18.0	18.3	18.5	18.8	19.9	18.7	16.0
	High 3rd	18.6	17.1	17.8	17.6	18.0	18.7	18.8	19.4	19.5	19.9	21.1	17.7
Maternal education	≤Middle school	3.9	6.7	6.2	5.3	4.7	3.9	3.0	2.6	2.2	1.9	1.6	1.6
	High school	47.6	56.8	54.9	53.4	52.5	50.4	46.8	43.5	41.8	38.9	35.5	33.3
	≥College	48.5	36.5	38.9	41.3	42.8	45.7	50.2	54.0	56.0	59.2	62.9	65.0