Hello dear Statalisters!

I am a student doing my Bachelor thesis and I am new to Stata as well as to this forum. I would deeply appreciate help from you guys with some issues I have got. I am studying Import flows to the EU from 155 exporting countries using a Gravity Model. I study trade flows for two years. My idea is to run a PPML and OLS regression and compare their results. My main variables to study are "Documentary compliance (Hours)" and "Border compliance (Hours) for the exporting countries. The data is taken from World Bank's Doing Business Database.

When running the OLS regression I get the result that Documentary compliance is significant and Border compliance is insignificant for Imports to the EU. The coefficient for Doc compliance is -0,35 and Bord compliance is -0,02. The equation that is used is:
Code:
regress lnImports lnDocCompExpHours lnBordCompExpHours lnGDPImp lnGDPExp lnGDPPCImp lnGDPPCExp lnDistance PTA Landlocked Commonofficiallanguage Historicalcoloniallink contiguous Remoteness Yeardum1, vce(robust)
As I don't want my dummies to be omitted, instead of using the
Code:


xtreg…, fe



I use the one i mentioned above. As I understand this is okay and doesn't make my results non-valid?



However, I struggle when doing a PPML regression, to overcome the issue with observations that take the value zero. I have been trying to find out which PPML command to use, as there seem to be many options. The ones I have tried are



Code:


xtpoisson


and



Code:


ppml


. First of all I would really appreciate if you have any suggestions on what PPML method that is appropriate. Also, as far as I know, when doing the regression you should log every continuous variable apart from the dependent one. Therefore I have tried both the -ppml- command as well as the -xtpoisson- command together with



Code:


,vce(robust)


in the end for xtpoisson only as it was not possible together with the ppml command. When running the ppml with the same code as above (apart from the dependent variable not being logged) the majority of the included variables are suddenly insignificant, including both Border and Documentary compliance.

When running the xtpoisson the results are different from the OLS regression. Both Border compliance and Documentary compliance are significant. However, the results are the opposit from the OLS as Border compliance has the coefficient -0,33 and Documentary compliance is -0,06. What does this say about the robustness of our results when three different methods (OLS, ppml and xtpoisson) give such different results?



To summarize, xtpoisson has significance for the two variables of interest, ppml for none and OLS for Documentary compliance but not Border compliance. How should/could this be interpreted? Do I do anything wrong?



I am sorry if I am not clear enough and if I have written this post incorrectly. If you want me to specify my issue better or differently I happily do so.



Best regards,



Mårten