[Rasch] Unidimensionality (follow up with the practical issue)

Trevor Bond trevor.bond at jcu.edu.au
Sat May 10 16:28:19 EST 2008

Dear steve et al,

Let's start with the final pre-supposition:
But using a Rasch model this way would require treating the data as 
though it were unidimensional, even though we have strong reason to 
believe that it is not.

We should treat the data as unidimensional only to the extent that we 
have evidence to support that for the purpose of the decisions we 

Different aspects of a (maths) test do not (necessarily) imply 
several measurement dimensions.

That's where we got into this discussion with the Ferguson ref 
claiming factors can be artifactual (artifacts of the difficulty 
range) consequences of the FA (correlations based) analyses. (see 
ppxii-xiii of B&F 2001 for details)

So that is why I need better to understand what happens when you 
extract a Rasch Dimension from what is truly a multi-dimensional 
Multidimensions are perhaps better analysed using the MCMLM model 
used for PISA (B&F 2007, pp258-260).

Then we get back to the criticism I have of much educational testing: 
if mere exposure to test items invalidates their use / changes their 
test characteristics, I wonder what they really test. My guess it is 
not understanding of the maths concepts.

Translator's note:
maths in Oz = math in US

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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