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Re: AW: st: question


From   vwiggins@stata.com (Vince Wiggins, StataCorp)
To   statalist@hsphsun2.harvard.edu
Subject   Re: AW: st: question
Date   Tue, 03 Nov 2009 11:28:31 -0600

Tunga Kantarci <tungakantarci@hotmail.com> asks why, after estimation with
-regress-, -nlcom- will not work when he references values such as e(rss) and
r(df_r) in his expression.

> I wish to calculate a figure where I call coefficient estimates and
> residuals sum of squares and degrees of freedom. All these
> quantities are from the stata estimation output. When I run the
> relevant command
>
>     . nlcom exp(_b[_cons]+_b[income]*15.3)*exp(e(rss)/e(df_r)/2)
>
> I get the error "could not calculate numerical derivative". This is
> perhaps because e(rss) and e(df_r) are not estimates. (As Maarten
> pointed out yesterday actually)

The error message could be improved (and we will look into that), but the
problem is that Tunga's expression contains the estimated results e(rss) and
r(df_r), but -regress- does not estimate the standard error of those estimates
nor their covariance with _b[_cons] and _b[income].  Without those standard
errors and covariances, -nlcom- cannot estimate a standard error for Tunga's
expression.  We could argue whether e(df_r) is an estimate, but not e(rss).
-nlcom- treats all e-class results as estimates.

If Tunga does not need an estimate of the standard error then he can heed 
Martin Weiss' <martin.weiss1@gmx.de> advice and type,

>     . di exp(_b[_cons]+_b[income]*15.3)*exp(e(rss)/e(df_r)/2)

If Tunga wants standard errors and wants to treat e(df_r) as fixed (OK by me)
and also treat e(rss) as fixed (that gives me pause), then he can macro-expand
their values so that they become constants in his expression.

      . nlcom exp(_b[_cons]+_b[income]*15.3)*exp(`e(rss)'/`e(df_r)'/2)

-nlcom- now sees these two values as constants and not as something from e().


-- Vince
   vwiggins@stata.com

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