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  • How can I include correctly asimetric country-pair fixed effects in "ppmlhdfe" comand?

    I hope you have a good day
    I would like to please answer the following questions
    How can I include asymetric country-pair fixed effects in the "ppmlhdfe" command?
    Previously I have built the so-called pairs of asymmetric countries and the pairs of symmetric countries with the following formula:

    *Asimetric country-pair
    egen pair_id_asimetricos=group(iso3_o iso3_d)
    replace pair_id_asimetricos = pair_id_asimetricos +100000 if iso3_o==iso3_d

    I have run the following regression:

    ppmlhdfe tot_trade_millon ln_pib_o_ ln_pib_d_ ln_gdpcap_o ln_gdpcap_d ln_REM_EXP_1 ln_REM_IMP_1, absorb(pair_id_asimetricos) cluster(pair_id_asimetricos)

    What is striking is the following:

    Absorbed degrees of freedom:
    Absorbed FE | Categories - Redundant = Num. Coefs |
    pair_id_asimetricos | 25652 25652 0 *|
    * = FE nested within cluster; treated as redundant for DoF computation

    Why does it show me that my asimetrics pair fixed effects are redundant ?
    why does it show me "num coef iqual to 0"

    Thank you for yours answers

    best wishes