-egen- is going to calculate
a mean over missing and non-missing
values of its argument alike. The
mean so calculated will be the mean
of the non-missings but it will
be attached to missings too.
You can see for this for yourself
by e.g.
. sysuse auto
. egen mean = mean(rep78), by(foreign)
. edit foreign rep78 mean
Your d2 variables should set things
straight again, however. Whenever
a response is missing,
response - mean = missing - mean = missing
irrespective of what is in mean.
The explanation lies elsewhere, I guess.
Is -student- ever missing?
Nick
[email protected]
Tim R. Sass
>
> I am trying to "manually" compute a fixed-effects estimator
> by taking the
> differences from means of all variables and then running reg on the
> demeaned data. You may ask why in the world I would want to
> do that, but
> that's for another post.
>
> I have a panel of student-level data over three years. I
> demean the data
> as follows:
>
> bysort student:egen nrtrgain_m = mean(nrtrgain);
> bysort student:egen charter_m = mean(charter);
> bysort student:egen nschools_m = mean(nschools);
> bysort student:egen chgschl_m = mean(chgschl);
>
> gen d2_nrtrgain = nrtrgain - nrtrgain_m;
> gen d2_charter = charter - charter_m;
> gen d2_nschools = nschools - nschools_m;
> gen d2_chgschl = chgschl - chgschl_m;
>
> I then run the following models:
>
> areg nrtrgain charter nschools chgschl,
> absorb(student) ;
>
> reg d2_nrtrgain d2_charter d2_nschools d2_chgschl ;
>
> xtdata nrtrgain charter nschools chgschl, fe clear;
> reg nrtrgain charter nschools chgschl;
>
>
> The first and third models yield the same estimated
> coefficients (except
> for the constant, of course), but the coefficients for the
> second model
> (using reg on the demeaned variables) yields different
> results. However,
> when I eliminate all observations with missing values for any of the
> variables in the model, all three models yield identical
> estimated slope
> coefficients.
>
> I'm guessing the problem has something to do with how egen
> computes the
> mean for each student when there are missing observations. I
> have read
> through the manual and searched the archives, but still can't
> figure out
> what is going on. Any help would be greatly appreciated.
>
> Tim
>
>
> Tim R. Sass
> Professor Voice: (850)644-7087
> Department of Economics Fax: (850)644-4535
> Florida State University E-mail: [email protected]
> Tallahassee, FL 32306-2180 Internet:
http://garnet.acns.fsu.edu/~tsass
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