Liu, Xianchen
Liu, Zhen-Zhen
Fan, Fang
Jia, Cun-Xian
Funding for this research was provided by:
National Natural Science Foundation of China (81573233)
Article History
Received: 30 January 2018
Accepted: 16 May 2018
First Online: 26 May 2018
Compliance with ethical standards
: We obtained permission to conduct the study from the principals in the target schools and informed consent from participants in the target classes before the survey. Participants were asked to get permission from their parents to participate in the survey. The study was approved by the research ethics committee of Shandong University School of Public Health and target schools.