I run a gravity model regression using PPML and fixed effect for the full period 1980-2020, subperiod 1 1980-2000, subperiod 2 2001-2020. In all cases the number of countries (N) is 15. My key variables of interest are: (i) currency union dummy (CU) which is 1 if the countries are in a currency union and 0 otherwise, (II) Free trade agreement (FTA) dummy which is 1 for all years after the free trade agreement was signed and 0 for all years before the agreement was sign.
My problem is for subperiod 2 either one or both variables of interests are omitted with no estimates.
The help I need: Is there any way I could write the command to ensure both variables of interest are estimated please?
I look forward to your help and I thank you in advance.
My problem is for subperiod 2 either one or both variables of interests are omitted with no estimates.
The help I need: Is there any way I could write the command to ensure both variables of interest are estimated please?
I look forward to your help and I thank you in advance.
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