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st: Panel data, missing values and estimation in differences.

From   "Diego Rei" <>
To   <>
Subject   st: Panel data, missing values and estimation in differences.
Date   Thu, 06 May 2004 11:54:02 +0200

Hello to all,
I have two questions regarding panel data estimation.
I am running two regressions with the within estimator, once inserting the dummy variables, the other with the differenced series. Since I get different results for certain coefficients, I was trying to understand if this can depend, beside some theoretical problems (endogeneity, errors of measurment etc), from the changed size of the sample. I have 18 countries in my sample, and the differences in the number of observations between levels and differences is 18. Intuitively  Stata should eliminate the first year. But if I run the regression in levels excluding the first year (if != option)the number of observations does not change!So,:
- Is there a way to understand what observations does Stata run the regression on in the difference equation and run the levels one on the same ones?
Apart from this, I have a probably elementary question that is troubling me. 
-If a variable is constant in levels for a certain country(and thus captured by the dummy) , hence assuming values 0 in differences, does this country participate in the estimation of the coefficient of that variable in the difference equation?

Thanks a lot for any help you may be able to give me, and sorry for the lenght of the message.
Have a good day

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