[Rasch] Unidimensionality

Trevor Bond trevor.bond at jcu.edu.au
Thu May 8 09:12:04 EST 2008

>Dear Antony

what comes first, the measure or the residuals ?

>In a couple of papers I noticed that the researchers before using 
>Rasch model or IRT models first use factor analysis to ascertain 
>unidimensionality. Since unidimensionality is a prerequisite to use 
>IRT models. They give the impression that only after FA has shown 
>that the test is unidimensioanl, one can use Rasch or IRT models.
>Is this really necessary? Isn't the Rasch model itself a technique, 
>superior to FA, to demonstrate unidimensionality?

my view is:
1 find and remove the Rasch dimension
2 use FA of the residuals to determine if sufficient structure exists 
in those residuals to infer more than one dimension in the data - the 
residuals should be randomly distributed.

For mine: FA used before RM misses the point of RM
Trevor G BOND Ph D
Professor and Head of Dept
Educational Psychology, Counselling & Learning Needs
D2-2F-01A EPCL Dept.
Hong Kong Institute of Education
10 Lo Ping Rd, Tai Po
New Territories HONG KONG
Voice: (852) 2948 8473
Fax:  (852) 2948 7983

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