st: Observation number in fixed effect regression

["Hui Wang" <jackie_stata@hotmail.com>]

Hi all,

I want to run the following regression:

Y = pai0 + pai1*A+X*pai2+e

Each observation is a kid in certain households. 'A' is the variable I am 
interested in; "X" is a vector of other controls. There are totally 1100 
kids. Some of them are siblings in the same houshold. The number of kids who 
have siblings in the dataset is about 200. If I run the following command in 
stata:

xtreg Y A X , fe i(id)

where id is the houshold ID. I should expect the stata to explore the 
variation within the households with multiple kids to estimate pai1. That 
says, the observation number should be about 200. But in the results, stata 
report that the number of observation is actually about 1100 (the whole 
sample).

Why is that? Is there any way to correct this? I found that the same problem 
does not happen to  logit regression (xtlogit).

Thanks,

Jackie

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