Hello all,

I am a masters student using the gravity model to try and understand trade between the former Soviet Union States between 1992-2014. I've built a composite dataset using gravity data from Head and Mayer, trade data from the Correlates of War Project, and democracy data from both Polity5 and v-Dem.

As I've researched, I discovered that the PPML estimator is commonly used to account for zero trade flows (among other reasons). I am attempting to run a PPML regression with fixed-effects dummies. These include 'exporter-time', 'importer-time' and country pair dummies. Unfortunately I am encountering a problem whereby the coefficient for distance is estimated. This should be dropped the regression due to collinearity with the dummies. Can anyone help address this? I can provide any details regarding data and commands I have used.

Code:
ppml Flow_Trade origin_time_* destination_time_* Pair_* revised_polity_o revised_polity_d eu_o eu_d CIS_o CIS_d log_distance, cluster(panel_id)
I have read as much as I could find already on Statalist about this issue, have tried running the regression with distance in various positions, however Stata is dropping multiple fixed effects dummies instead. I found this strange because the OLS regression I am also using drops the distance variable.

Thanks in advance for any assistance.