Re: st: sigma u listed as zero despite unit variance

[Constantine Daskalakis <C_Daskalakis@mail.jci.tju.edu>]

At 11:14 AM 2/24/2004, Amber E. Boydstun wrote:
> Hi Mark,
>
> Thanks for your help!  You're right that when I run normal OLS I get the same
> results (posted below).  Can you tell me, is the reason that the random 
> effects
> estimator has degenerated into the OLS estimator that I have such a small 
> range
> of variance of mean dependent variable values across my units (from 
> .6603999 to
> .6897267)?  I guess I just thought that this small range would still count as
> SOME amount of random error that would register as a very small sigma_u, but
> something slightly greater than zero.  Thanks again.
>
> - Amber
The variability you're referring to is the margninal (unconditional) 
variability of the outcome. It is small to begin with, and once you 
condition on the variables you have in the model, there's essentially no 
leftover variability for a random effect. That would be my guess.






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________________________________________________________________

Constantine Daskalakis, ScD
Assistant Professor,
Biostatistics Section, Thomas Jefferson University,
211 S. 9th St. #602, Philadelphia, PA 19107
   Tel: 215-955-5695
   Fax: 215-503-3804
   Email: c_daskalakis@mail.jci.tju.edu
   Webpage: http://www.jefferson.edu/medicine/pharmacology/bio/  
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