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