Methods Stratified cluster sampling was employed and 10 811 residents in Heyuan, Meizhou, Shenzhen, Zhanjiang, Chaoyang, Guangzhou, Qingyuan, Shaoguan were investigated in 2006.

• 方法2006年，采用分层整群抽样方法对河源、梅州、深圳、湛江、潮阳、广州、清远、韶关8个地区进行了抽样调查。