It is often sensible to treat items as random and not as fixed. A test item is usually only a sample of the kinds of items that could have been drawn to try and operationalize the trait under study. Other items could have been used. Traditional IRT estimation treats items as fixed, however.




-----Original Message-----
From: r-sig-mixed-models-bounces@r-project.org on behalf of Stuart Luppescu
Sent: Mon 6/8/2009 6:58 PM
To: Ken Beath
Cc: r-sig-mixed-models@r-project.org
Subject: Re: [R-sig-ME] Rasch with lme4
 
On ?, 2009-06-09 at 08:04 +1000, Ken Beath wrote:
> The model treats item as a random effect and should be a fixed effect.

Hmm. In Doran, Bates, Bliese and Dowling (2007), the authors treat the
item as random. 

[snip]
> Another question to ask is whether the Rasch model is appropriate. If
> an IRT is more sensible it would cause some problems with the second  
> model.

Sorry, but I don't understand this at all.

-- 
Stuart Luppescu -=- slu .at. ccsr.uchicago.edu        
University of Chicago -=- CCSR 
???????? -=-    Kernel 2.6.28-gentoo-r5                
Drusilla: How do you feel about eternal life? 
 Xander: We couldn't just start with coffee?  
 
 
 
 
 
 
 

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