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st: areg vs xtreg, fe


From   "jenny.montaldo" <jenny.montaldo@libero.it>
To   "statalist" <statalist@hsphsun2.harvard.edu>
Subject   st: areg vs xtreg, fe
Date   Mon, 4 Feb 2008 18:22:10 +0100

Dear all,

hi I write to ask you one question regarding the difference between areg and xtreg, fe. In particular I would like to understand the difference concerning the r squared. Indeed I use a fixed effects model to estimate the returns to education on a panel of individuals. I performed fixed effects estimates for two categories of workers, blue collars (which are many) and white collars. 
I found out interesting results. However, for blue collars the estimates of the r-squared with xtreg,fe are the following: 
r^2 within 0.07
r^2 between 0.002
r^2 overall 0.004
when I do the same estimation with areg, I obtain an r^2 of 0.80. a huge difference. 

I would like to understand why is there a so big difference, and which one I should report...

Another related question concern the estimates for white collars. The xtreg, fe have an overall r^2 of 0.27 (much better than the previous one). However when I try the areg command it says that the matrix is not positive definite (I use also some lagged values of my variable of interests in the estimation). 

Could anyone help me?
many thanks 
jenny


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