# st: Re: missing dummy variable

 From Kit Baum To statalist@hsphsun2.harvard.edu Subject st: Re: missing dummy variable Date Thu, 11 Oct 2007 06:54:23 -0400

John said

Thanks Kit, but the coefficient is the easy part! It's the confidence
intervals I'm scratching my head over. Is this something that can be
dug out of the e(V) matrix or the like?

But he seems to miss my prior mention of -lincom- in a couple of postings. If you can get -lincom- to do it, it will give you point and interval estimates. Of course you can calculate it from e(V): the variance of a sum is the sum of the variances plus twice the covariances of all (i,j), i<j. Again, for two dozen coefficients, that involves 24(25)/2 = 300 distinct covariances. Luckily -lincom- will do it with a bit of setup cost:

/*
sysuse auto
rename make desc
gen make = word( desc,1)
tab make
*/
xi i.make
ds _I*
foreach v of varlist `r(varlist)' {
local lc "`lc' -`v'"
}
xi3: reg mpg e.make
lincom `lc'
// This coefficient and its standard error, added to the grand mean
// estimated as the constant term in xi3, should match the constant term in
xi: reg mpg i.make

Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html

On Oct 11, 2007, at 2:33 AM, statalist-digest wrote:

```Thanks Kit, but the coefficient is the easy part!  It's the confidence
intervals I'm scratching my head over.  Is this something that can be
dug out of the e(V) matrix or the like?
```
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