Re: st: Separate intercept mixed model

["Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>]

Dear Bobby

  Thank you very much for your reply. I thought I was losing my "statistical mind" not 
being able to figure out how to fit this separate intercept model.

Thanks much!

Michael N. Mitchell
Data Management Using Stata      - http://www.stata.com/bookstore/dmus.html
A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html
Stata tidbit of the week         - http://www.MichaelNormanMitchell.com



On 2011-02-08 11.05 AM, Roberto G. Gutierrez, StataCorp wrote:
> Michael Mitchell<Michael.Norman.Mitchell@GMAIL.COM>  asks:
>
> > I am trying to estimate a mixed (multilevel) model using a separate
> > intercept approach, and am getting an error saying that the likelihood
> > evaluates to missing. Here is the model I am estimating with the error...
>
> > . webuse pig, clear
> > (Longitudinal analysis of pig weights)
> > . replace week = week - 1
> > (432 real changes made)
> > . * create 2 groups, trt and control
> > . generate trt = (id>  24)
> > . * make weight 10 pounds more for treatment
> > . replace weight = weight + 10     if trt == 1
> > (216 real changes made)
> > . * Model 0:
> > . * Enter trt as factor variable
> > . xtmixed weight ibn.trt c.week , nocons || id:
>
> > Performing EM optimization:
> > likelihood evaluates to missing
> > r(430);
>
> Michael then explains that this should work because he generates the
> indicators manually and then runs -xtmixed- with these instead.  He is
> right.
>
> Michael has discovered a bug in how -xtmixed- is mishandling the "ibn."
> factor notation for including all levels of a factor variable, including
> the base category.
>
> This will be fixed in the next ado update.
>
> --Bobby
> rgutierrez@stata.com
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