Dear Statalist,

I am trying to run gravity regressions on the impact of FTA's for different sectors and having problems with the tariff variables. I tried to estimate the regression using the standard comand for PPML:

ppml_panel_sg tradeflow FTA n_MFN, ex(exporter) im(importer) y(year) cluster(pair_id)

My problem is that Stata drops the tariff data with the note:

note: ln_MFN omitted because of collinearity over lhs>0 (creates possible existence issue)

I would be really thankful if someone could help me with this issue.

Thanks a lot.