ppmlhdfe vs xtpoisson

[Chiel Enk]

Hi everyone,

I'm writing my thesis on the impact of the withdrawal of GSP on the imports from Thailand to the US.
I have read at several posts that my results should be the same when I use ppmlhdfe or xtpoisson, however I get different results.

The following are the codes I used:

xtpoisson imports gsp_removal mfn i.month_year, fe vce(robust)
ppmlhdfe imports gsp_removal mfn, a(month_year) vce(robust)

Does anyone know how there can be differences within the results?

Kind regards,

Chiel van den Enk

[Joao Santos Silva]

Dear Chiel Enk,

With the fe option, the xtpoisson command automatically includes pair fixed effects, so the two commands are including different sets of fixed effects. To get the same results you need to absorb the pair fixed effects in the ppmlhdfe command.

Best wishes,

Joao