Hello,

I'm trying to estimate trade potential for the BRICS group of countries - as in, intra-BRICS trade potential - using a gravity model and the PPML estimator. I'm generating predicted values, and then calculating trade potential by (predict-actual). My dependent variable is exports (in $USD MN), while my independents are ln(gdp) of exporter and importer, standard gravity covariates and membership in RTAs. I'm using exporter-time and importer-year fixed effects. I run this for aggregate merchandise trade and then replicate it for several products of interest. I'm using COMTRADE and CEPII gravity database to source my data.

I have three questions here:
1. How can I test for endogeneity in the context of the PPML estimator?
2. How does one test for misspecification problems such as RESET test? As far as I know, RESET tests for whether a linear form of suitable for our regression analysis. Does it also work when we no longer use OLS (like in my case here)?
3. Should I run my regression on all the relevant products of interest in one combined regression? Will that provide me with some additional insight?


This is my code (after importing dataset):

Code:
egen exporter_year = group(exporter year)
tabulate exporter_year, generate(EXPORTER_YEAR_FE)
egen importer_year = group(importer year)
tabulate importer_year, generate(IMPORTER_YEAR_FE)

drop if exp<0

ppml exp lngdp_ex lngdp_im lndistw contig comlang_off comcol col45 rta EXPORTER_YEAR_FE* IMPORTER_YEAR_FE*

predict fitted
gen potential = fitted - exp
(252 missing values generated)
Many thanks for your time and attention. Let me know if I should provide some extra information.

Regards,
Saunok