Dear Statalisters,
I am trying to use the gravity model to study the impact of implementation of TRIPS in partner countries (dummy=1 if the partner country has implemented TRIPS in the particular year or otherwises 0) on the export of pharmaceuticals from USA to 120 other countries from 1990-2018. Therefore my dependent variable is exports from USA= f(GDPPC; Pop, Distance; Lang; FTA; TRIPS;
Landlocked; Exchange rate; Openness index). I have two questions regarding the same:

1) In order to do so can I use the ppml command? xtpqml drops variables such as distance and language since they are constant within the group and this is not very ideal for my analysis.
2) Also, how do I check for endogeneity and autocorrelation while using ppml?

Thank you,
Sneha