[Rasch] Rasch & validity

Bond, Trevor trevor.bond at jcu.edu.au
Wed Jan 26 23:28:45 EST 2011


A number of us have written on Rasch links to Sam Messick's conceptions of validity:
you can see a flavour of it at B&F2 pp268ff
a few references are there for a more detailed follow up:
Fisher; Wilson; Smith; Bond.
NOTHING guarantees validity
BUT some aspects of Rasch measurement might help you present relevant evidence
IF you know what you are trying to measure.
Best
TGB

Prof Trevor G BOND
Adjunct Professor
School of Education
James Cook University
AUSTRALIA
mob: +61 416 82 70 83
________________________________________
From: rasch-bounces at acer.edu.au [rasch-bounces at acer.edu.au] On Behalf Of Svend Kreiner [S.Kreiner at biostat.ku.dk]
Sent: Wednesday, 26 January 2011 9:32 PM
To: rasch at acer.edu.au
Subject: Re: [Rasch] Rasch & validity

Anthony,

It depends on your definition of validity. I subscribe to Paul Rosenbaum's (Psychometrika, 1989)definition of criterion related construct validity requiring unidimensionality, monotonocity, local independence and no DIF. For this reason, measurement by items from Rasch models is valid because the Rasch models meet all these requirements.(plus, of course, a number of other requirements that we think are important - Rasch measurement is more than valid, it is also objective and sufficient).

If you think about criterion validity and/or Cronbach & Meehl's definition of construct validity you would not immediately think about Rasch models, but it follows from Rosenbaum's results that if items follow a Rasch model and if X is known to be positively related to the latent trait, then X will also be positively related to the score. If X is a criterion variable and X is not associated to the total score then it follows that items cannot fit the Rasch model.

Svend

Den 26-01-2011 11:37, Anthony James skrev:


Dear all,
Validity is defined as "the degree to which a test measures what it is supposed to measure".
How can fit to the Rasch model be evidence of validity?
Can't we have a say, anxiety scale that perefctly fits the Rasch model but does not actually measure anxiety?
How does Rasch analysis guarantee that a test actually test what it should test?

Cheers
Anthony



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