逐年PSM保存数据时提示某些年份没有值是为什么
file 2002.dta saved
(note: dataset contains 0 observations)
file 2003.dta saved
(note: dataset contains 0 observations)
file 2004.dta saved
(note: dataset contains 0 observations)
file 2005.dta saved
file 2006.dta saved