Table 1.

Characteristics of the eligible participants

Characteristic	n	%
Total	22421	100
Sex
Female	4263	19.0
Male	18158	81.0
Age group in year
18-29	6301	28.1
30-39	8818	39.3
40-49	3487	15.6
50-59	2244	10.0
60-69	1138	5.1
70-79	433	1.9
Hospital location
Beijing	6659	29.7
Chengdu	1048	4.7
Guangzhou	1348	6.0
Lanzhou	3771	16.8
Nanjing	9595	42.8