就是有的变量的数据是缺失的，比如有个变量var1数据都是1和0，你生成一个变量，当var1=1的时候，var2等于5，因为var1中有的变量为0，这部分为0的观测值在var2里就是缺失值了
mi语法简介：Stata中的missings数据处理说明书 Title intro—Introduction to mi Syntax £ To become familiar with mi as quickly as possible,do the following:1.See A simple example under Remarks below.2.If you have data that require imputing,see[MI]mi set and[MI]mi impute.3.Alternatively,if ...