Study cities and participants
Selection of the study cities and participants has been described
previously in detail.15 In short, the cross-sectional
study was conducted in 27 districts of seven northeast cities in China
from 2012 to 2013, including six districts in Shenyang, three districts
in Anshan, three districts in Benxi, five districts in Dalian, three
districts in Dandong, four districts in Fushun and three districts in
Liaoyang. In each district, one or two kindergartens, primary schools
and middle schools were randomly chosen. In each grade of the selected
school, one class was randomly chosen and all the students in the class
were enrolled. Totally, we obtained 68,647 participants aged from 0 to
20 years.
This study was carried out in accordance with the World Medical
Association Declaration of Helsinki Ethical Principles for Medical
Research Involving Human Subjects. The Human Studies Committee of Sun
Yat-sen University approved the study protocol and all the parents or
guardians of the participants signed the informed consent before
starting data collection.