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Re: st: Running an empty model using xtmixed


From   rgutierrez@stata.com (Roberto G. Gutierrez, StataCorp)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Running an empty model using xtmixed
Date   Fri, 28 Mar 2008 14:11:32 -0500

Satomi Yamamoto <syamamot@ad.uiuc.edu> asks:

> Could I use the xtmixed command to run a null model?  I used the pig.dta
> data (http://www.stata-press.com/data/r10/pig.dta) and run

> xtmixed weight||id:, var

> But, I was not able to converge the data:

[...]

> Is there another way of running the intercept only model in Stata?  Or, did
> I forget to include options in xtmixed? It would be great if somebody could
> help me wit this.

There is nothing that states you can't fit a null model with -xtmixed-, and
the syntax you use is the correct one.  It is just that in your particular
case -xtmixed- failed to converge, most likely because of a near-zero variance
component at the -id- level.

    . sysuse auto, clear
    . xtmixed mpg || rep78:

does converge.  In fact, you can even be more "null" and do

    . xtmixed mpg

or if you want to get really crazy

    . xtmixed mpg, nocons

--Bobby
rgutierrez@stata.com
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