st: correction variance- cov matrix panel data sample selection

[ncdcta00@uniroma2.it]

Dear statalist, Do you know some routine or reference for correction  
the standard error and variance matrix in panel data sample selection?
I calculate the inverse Mills' ratio every year, and after I put this  
in a simple regression for correctin the selection, but standard error  
and coefficientes are bias, so I need to corecct them.

Thanks for your help.
Best,
Catia

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/