我用的聚类稳健标准差，传统的Hausman检验不适用。按照陈强书中的步骤做，做到最后一步就出错了，这是怎么回事？求大神指导！
quietly xtreg ofdi pgdp gdp freedom resourse facility corrupt distance exchan
> ge,re

. scalar theta=e(theta)

. global yandxforhausman ofdi pgdp gdp freedom resourse facility corrupt distan
> ce exchange

. sort cntry

. foreach x of varlist $yandxforhuasman{
  2.
.   2. by cntry:egen mean`x'=mean(`x')
  3.
.   3. gen md`x'=`x'-mean`x'
  4.
.   4. gen red`x'=`x'-theta*mean`x'
  5.
.   5. }
varlist required
r(100);