choose excluded variables

[oznur ucar]

Hi, I have 2 dummy variables groups and 6 dummy variables in my panel. I want to choose excluded variables. How can ı do? My used formula
"xtabond2 x lx y z_uk z_fr z_ge z_sc t_eu eu , twostep robust nomata iv( y z_uk legor_fr z_ge z_sc t_eu eu ) gmm( lx, lag(1 12) collapse)

thank you
Öznur