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st: xtpoisson clustering

From   Mila Kashcheeva <>
Subject   st: xtpoisson clustering
Date   Mon, 7 Oct 2013 17:06:25 +0800

Dear Statalist,

I work with a panel datatset where I regress firm-country imports on a
number of parameters. I use xtpoisson function to be able to account
for zero imports, use fixed effects and cluster standard errors. I
want to include country, year and firm fixed effect and at the same
time cluster standard errors by country.

The xtpoisson command allows clustering only at a level of fixed
effects included via "fe i(variable) vce(robust)" , and since I want
to cluster by country I include ",fe i(id_country) vce(robust)" and
also include year fixed effects and firm fixed effect manually in the
regression. The regression calculates the estimates for all my main
variables and for the year fixed effects, but does not calculate
standard errors for the firm fixed effects and instead gives me dots
for most of the firm fixed effects (with just a few standard errors

Can someone explain why the xtpoisson command does not calculate
standard errors for firm fixed effects and whether I can trust the
rest of fixed effects in this regression.

Thank you for you suggestion!

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