st: Standard deviations from OLS fixed effects regressions

[Chris <chris.stata@gmail.com>]

Dear Statalist,

I want to calculate summary statistics for two groups after
controlling for heterogeneity via fixed effects. Groups are denoted by
dummy D=1 and D=0. To do this, I run

xtreg depvar D, fe

Now the mean (after controlling for fixed effects) is just _cons (for
D=0) and _cons+_b[D] (for D=1). Clearly, standard errors are reported,
but I have no idea how to get the SD for each of the two group means.
(Just multiplying the SE of the constant with the number of
observations from the D=0 group doesn't give the right answer; I
checked without FE, i.e., reg). Using predict, one can obtain the mean
by averaging predictions over all observations of the group, but I can
hardly use

predict se, stdp

and average standard errors of individual predictions. But how else am
I going to get those SDs? Any help is very much appreciated.

(I posted this question also here with a more detailled description,
but I believe they are mostly using R:
http://stats.stackexchange.com/questions/60991/retrieving-standard-deviations-from-ols-estimates)
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