Methods A cluster stratified sampling method was used to choose 3 251 students from 60 classes in 10 middle schools, among which, 3 044 questionnaires were available.
方法按照分层随机整群抽样的方法，抽取蚌埠市10所中学60个班级的3 251名高中生为研究对象，获有效问卷3 044份。
Methods A cluster stratified sampling method was used to choose 3 251 students from 60 classes in 10 middle schools, among which, 3 044 questionnaires were available.
方法按照分层随机整群抽样的方法，抽取蚌埠市10所中学60个班级的3 251名高中生为研究对象，获有效问卷3 044份。
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