Funding for this research was provided by:
Scientific Research Fund of National Health Commission of the People’s Republic of China (201502002)
Text and Data Mining valid from 2020-01-08
Received: 24 August 2019
Accepted: 24 December 2019
First Online: 8 January 2020
Ethics approval and consent to participate
: This study protocol was approved by the Stomatological Ethics Committee of the Chinese Stomatological Association, Beijing (No. 2014–003). A written informed consent form was sent to the parents of each child to explain the content of the survey. Each parent returned a signed.informed consent form.
: Not applicable.
: The authors declare that they have no competing interests.