Procedure
The investigation was conducted on a voluntary, confidential, and anonymous basis, and the participants could withdraw at any time during the study. In each of the 4 hospitals, two specially trained nurses were appointed to conduct face-to-face investigations of the participants. The questionnaires were completed by the participants in a quiet and private room. For children with difficulties in reading or comprehension, the investigators assisted them to complete the questionnaire on the basis that they fully understood the content and made their own decisions for each item. Before collection of the questionnaires, the participants were asked to re-check for omissions.