*更多期滚动(不生成大量变量):
https://bbs.pinggu.org/thread-1164714-3-1.html
*直接生成方差
egen id=group(stkcd)
su id
set ob `=_N+r(max)'
replace year=2012 if id==.
bys id (year): replace id=_n if id==.
xtset id year
tssmooth ma s=roa,w(5)
tssmooth ma s2=roa^2,w(5)
by id: g var=(s2[_n+1]-s[_n+1]^2)*5/4 if _n>4
drop s s2
drop if year==2012
补充内容 (2013-6-17 02:08):
*可简作:
egen id=group(stkcd)
xtset id year
tssmooth ma s=roa,w(4 1)
tssmooth ma s2=roa^2,w(4 1)
by id (year): g var=(s2-s^2)*5/4 if _n>4