教育有多个分类，减少分类，recode 教育，教育分成basic    secondary  higher 然后检查新的变量是否有缺失

1 - tertiary
2 - unfinished tertiary
3 - special secondary
4 - secondary
5 - 7 years of schooling 6 - primary
7 - can read and write 8 - can read
9 - illiterate
10 - child aged 0-8
99 - missing value