Richard, Austin, and (hopefully) Jeff,
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of
> Austin Nichols
> Sent: 13 September 2007 09:13
> To: [email protected]
> Subject: Re: st: suest with large number of fixed effects
>
> Richard Boylan <[email protected]>:
> Interesting. I used a not-up-to-date Stata 9.2 (20 Jan 2006)
> and the example ran fine (danger of not being able to use
> -update qu- with a nonstandard winsock.dll, and being
> forgetful, I suppose), but produced absurdly low p-values
> (i.e. absurdly small SEs relative to the individual
> regressions, esp with -cl(id)- corrections). After a manual
> update (to 20 July 2007), I got the error message you report,
> but I don't see the relevant change in -help whatsnew-
> anywhere (a post to Statalist by Jeff Pitblado on June 27,
> 2006, indicates it probably happened in the 6 July 06 update
> which was also the MP release).
>
> Mark Schaffer gives a way forward by manually demeaning, but
> I suppose any concerns Stata has about applying -suest- to
> -areg- would apply to any such -reg c_y c_x- procedure?
I had forgotten about Jeff's post. Here is the key extract:
"We've recently discovered that -suest- yields incorrect results when
used
after -areg-. This is not something that can easily be fixed given that
the
meat of the sandwich estimator of variance (Robust/Huber/White VCE)
cannot be properly computed due to the fact that the coefficient
estimates for the
absorbed categories in -areg- are not present in -e(b)- (and the
corresponding indicators are not present in the dataset)."
and here's a link to the full post:
http://www.stata.com/statalist/archive/2006-06/msg00874.html
What puzzles me about Jeff's statement is that absorbing
(partialling-out, demeaning, removing through first-differencing) the
fixed effects is no obstacle to using the
sandwich/robust/Huber/White/and-don't-forget-Eicker VCE with other
estimators.
For example, demeaning and using -regress- with -cluster- generates
exactly the same results as -xtreg,fe cluster()- and indeed -areg-, as
this example shows (again using Ben Jann's -center- command):
sort id
by id: center ys k n, casewise
reg c_ys c_k c_n, cluster(id)
xtreg ys k n, i(id) cluster(id) fe
areg ys k n, absorb(id) cluster(id)
If it's legitimate to use the robust VCE with transformed individual
equations, shouldn't it also be legitimate to use -suest- to combine the
equations? The "within-equation" parts of the VCE reported by -suest-
will be the same (though maybe with a different and asymptotically
uninteresting dof adjustment) as that obtained by estimating the
separate equations on their own. -suest- is just adding the
"cross-equation" part of the VCE.
I'd be very interested in hearing more about this from Jeff or anyone
else.
Cheers,
Mark
> Might you perhaps compare the manual approach using demeaned
> data to one using the two-way clustering approach of Cameron
> Gelbach and Miller (paper at
> http://nber.org/papers/t0327 and code at
> http://glue.umd.edu/~gelbach/ado/cgmreg.ado) on a stacked
> dataset? In any case, I think there is good reason to prefer
> (to the -suest- approach you'd like to use, clustering on obs
> across panels, and on panel across obs) the SEs you get from
> the individual regressions you
> specified:
>
> xtreg y1 x1, i(id) fe cl(id)
> xtreg y2 x2, i(id) fe cl(id)
> xtreg y3 x3, i(id) fe cl(id)
>
> is it true you don't have any common regressors across equations?
>
> On 9/12/07, Richard Boylan <[email protected]> wrote:
> > Thanks, but I should have mentioned that a previous post on the
> > statalist discusses how areg cannot be not be used with
> suest because
> > those estimates are incorrect. So, if I have to assume that you are
> > using an older version of STATA (7 or older) that allows you to use
> > suest with areg.
> >
> > In the newer version of one tries to do that one obtains
> >
> > areg is not supported by suest
> >
> >
> > On 9/12/07, Austin Nichols <[email protected]> wrote:
> > > Richard Boylan--
> > > Indeed -areg- will mechanically give you an answer when combined
> > > with
> > > -suest- but you should be aware that the cluster-robust estimator
> > > can give downward-biased estimates of the true standard
> deviation of
> > > your estimates, so a smaller p-value after -suest- may be
> suspect.
> > > Also, just to be clear, -suest- will not give you more precisely
> > > estimated coefficients since it will not change your
> estimated coefficients.
> > > Under some circumstances it will give you better estimates of the
> > > standard errors, and those circumstances include having a large
> > > number of clusters and observations. How large? That depends...
> > >
> > > webuse abdata
> > > areg ys n w k, a(id)
> > > est sto ys
> > > areg wage n w k, a(id)
> > > est sto wage
> > > suest wage ys, cluster(id)
> > >
> > > On 9/12/07, David Jacobs <[email protected]> wrote:
> > > > The command "areg" is designed for this purpose, but
> I'm not 100%
> > > > sure that it has a score option or that it's matrix
> isn't equally large.
> > > >
> > > > Dave Jacobs
> > > >
> > > > At 11:11 AM 9/12/2007, you wrote:
> > > > >I would like to estimate several regressions separately, but
> > > > >using suest to obtain more precisely estimate coefficients
> > > > >
> > > > >So, what I would like to do is:
> > > > >
> > > > >xtreg y1 x1, i(id) fe
> > > > >est store eq1
> > > > >xtreg y2 x2, i(id) fe
> > > > >est store eq2
> > > > >xtreg y3 x3, i(id) fe
> > > > >est store eq3
> > > > >suest eq1 eq2 eq3, cluster(id)
> > > > >
> > > > >Given that xtreg does not have a score option, it is
> discussed in
> > > > >previous postings that one needs to estimate the model using a
> > > > >linear regression with dummy variables.
> > > > >
> > > > >The problem I have is that I have 1000 fixed effects
> and thus the
> > > > >matrix computed in suest is going to be way too large.
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