[Rasch] One model for three constructs and four groups (was RE: PCA and DIF)

Stuart Luppescu slu at ccsr.uchicago.edu
Wed Jun 8 01:05:36 EST 2011

On Sun, 2011-06-05 at 19:28 -0700, Juanito Talili wrote:
> My question is: What does it mean when DIF test is significant for
> several items  and nonsignificant for other items?  Can I still use
> the instrument for the four shops in the future ?

This is a tricky question. You have to balance the amount of
mis-measurement with your desire to compare measurements across shops.

In our work, we survey elementary school and high school teachers and
students. Very often high school teachers score lower than elementary
school teachers and show DIF on certain items. If the DIF is really
large we sometimes make different versions of some of the items for
elementary and HS. But too much of this obscures the differences in the
measures. If we make completely different elementary and high school
versions of the measures we get much better item fit, but then we can
not make comparisons among all schools, since elementary schools will be
measured only relative to other elementary schools, and high schools
only to other high schools. In this case, the mean item difficulties for
elementary schools will be 0, and high schools also 0, so elementary vs.
high school differences in teacher and student measures will be

In the same way, if you let each shop have it's own item difficulties,
you will not be able to measure between-shop differences.
Stuart Luppescu -=- slu .at. ccsr.uchicago.edu        
University of Chicago -=- CCSR 
才文と智奈美の父 -=-    Kernel 2.6.38-gentoo-r6                
As for elegance, R is refined, tasteful, and
 beautiful. When I grow up, I want to marry R.   
 -- Andy Bunn (in a discussion about whether R is
 simple, powerful and       elegant)       R-help
 (May 2005)  
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