Hello all,

I have a dataset of 4789 observations (unbalanced 194 groups and 25 years) . The way this dataset is constructed, there are 133 groups (3325 obs) with all zero outcomes out of the 194 groups.

My predicament is that when using
Code:
xtreg y x, fe
all the 4789 observations are retained. However, on using
Code:
xtpoisson y x, fe
the 133 groups (3325 obs) with all zero outcomes are dropped and only 1464 observations (61 groups) are used to generate the estimates.

Please can you guide me on why this is happening. I am using STATA 15.

Thank you in advance!