Hi,
I am trying to estimate this gravity equation:
ppml trade PAIR_FE1-PAIR_FE$NTij_1 EXPORTER_TIME_FE* IMPORTER_TIME_FE1-IMPORTER_TIME_FE$NT_yr RTA, noconst
however, I have this error: "Warning: variance matrix is nonsymmetric or highly singular"
What does it mean? and how can I correct it?
Thank you!
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