Hi everybody

I have a problem with a heckprobit model. It runs perfect if i don´t consider one variable (age*age that is P60402) in the selection ecuacion; but if i include this variable then i have the error message: convergence not achieved (even using difficult). I wouldn´t like to eliminate this variable because the theory behind is very strong so, what can i do to avoid that solution?

This is the model:

heckprobit Y i.P6020 P6040 P60402 LOGP6800 i. P6070COPY LOGINGLABO i.P6220COPY i.P7020COPY i.P6430COPY P6426 P64262 i.P515COPY i.P514COPY i.P6870COPY i.P6880COPY2 [pw= fex_c_2011] , select (pea= i.P6020 ESC ESC2 P6040 P60402 P6070 INGRESTOFLIAPERC P6050COPY P6020#HIJOSMENORES) vce(robust) difficult

Thanks