1. sampling method: stratified random cluster sampling method is still adopted this year, that is, stratified by grade and randomly sampled by class to form monitoring samples.
2. Total number of samples: limited to the number of effective samples that can meet the minimum monitoring.
3. Sample grouping: 18-20-year-old Han students are divided into four categories according to urban, rural, boys and girls, with one-year-old students in each category.