st: xtpoisson, fe: 42 groups (93 obs) dropped because of all zero outcomes! Why?

["ippab ippab" <ippab.statalist@gmail.com>]

Hi Everyone,

I am running a fixed effects model with "xtpoisson, fe" command.  I am
trying to understand why the software is dropping a few observations
because of the following reason: "42 groups (93 obs) dropped because
of all zero outcomes".   More specifically, I am confused with the
term all zero outcomes.  How is it determining this 42 groups?  My y
variable is a count and includes zeros for more than 42 groups.
Clearly, it is dropping a subset of the groups with zero y variable.
May be the answer is quite trivial; unfortunately, I am not getting
it.  I would really appreciate any help.

Thanks a lot.

ippab
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