Linear fixed-effects-models can be understood as models on the deviances of the means, where the fixed-effect is the individual heterogeneity in the mean. Very often you are not interested in the deviance of a specific observation of a unit across time. All observation within a unit treated equally, i.e. they are symmetric with respect to time. Time becomes important, when you have/want to model correlation across time in the error term (help xtreg##correlation). I thing all of these options need the additional time specification in the xtset, but not the xtreg itself.
Klaus
__________________________________
Klaus Pforr
GESIS -- Leibniz Institut für Sozialwissenschaft
B2,1
Postfach 122155
D - 68072 Mannheim
Tel: +49 621 1246 298
Fax: +49 621 1246 100
E-Mail: klaus.pforr@gesis.org
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-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Justina Fischer
Gesendet: Donnerstag, 8. November 2012 11:27
An: statalist@hsphsun2.harvard.edu
Betreff: Re: st: AW: Question on xtreg and the FE option
How can stata use xtreg, fe without knowing the time dimension?
-------- Original-Nachricht --------
> Datum: Thu, 8 Nov 2012 10:21:07 +0000
> Von: Nick Cox <njcoxstata@gmail.com>
> An: statalist@hsphsun2.harvard.edu
> Betreff: Re: st: AW: Question on xtreg and the FE option
> This is not correct. There is no rule that a time variable _must_ be
> specified.
>
> On Thu, Nov 8, 2012 at 10:11 AM, Justina Fischer <JAVFischer@gmx.de>
> wrote:
> > Hi Andrew
> >
> > when using xtset you have to specify both observational unit and
> > time
> dimension at the same time.
> >
> > E.g. xtset country year
> >
> > or xtset id timeline
> >
> >
> > etc.
> >
> >
> > Best
> >
> > Justina
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