[Rasch] DIF test with 2-item scale

Thomas Salzberger thomas.salzberger at gmail.com
Wed Jan 4 10:56:24 AEDT 2017


Trevor,

Given that invariance is an integral part of the Rasch model and that DIF
is a violation of invariance, by implication DIF is a violation of the
Rasch model in my mind.
But, of course, it is a violation that - technically - we can correct for.
When one should do so, and if so, how, is another question.

Best
Thomas

___________________________________
Dr. Thomas Salzberger
WU Wien University of Economics and Business (Austria)
Modus Outcomes - Psychometric Consulting Associate
MBC Matilda Bay Club
Thomas.Salzberger at gmail.com
Thomas.Salzberger at wu.ac.at


2017-01-03 23:46 GMT+01:00 Bond, Trevor <trevor.bond at jcu.edu.au>:

> Dear Svend,
> Is DIF 'evidence against the Rasch model' or evidence that the data do no
> conform to its requirements for measurement? Perhaps a tendentious
> question, but I seek clarity for me, and perhaps, others who lurk on the
> forum.
> Collegially
> Trevor
>
> Prof Trevor G BOND
>
> > On 4 Jan 2017, at 2:12 am, Svend Kreiner <svkr at sund.ku.dk> wrote:
> >
> > You can (and should) test for dif. If evidence of dif turns up, it is
> evidence against the Rasch model. The problem, as Thomas has already said,
> is that you have no way to determine whether one of the items have no dif.
> >
> > Also, you would have to have many more items fitting a Rasch model to
> convince anyone that you have a valid measure of anything. Three items are
> better. In such cases you can meaningly do a statistical analysis of the
> fit to the Rasch model and the claims of both no dif and local
> independence, but what about the non-statistical requirement of content
> validity? That would require many more items, I think.
> >
> > Svend
> > ________________________________________
> > From: Rasch [rasch-bounces at acer.edu.au] on behalf of Randy and Shelley
> MacIntosh [srmac at fastmail.com]
> > Sent: Monday, January 02, 2017 8:05 PM
> > To: rasch at acer.edu.au
> > Subject: [Rasch] DIF test with 2-item scale
> >
> > I have several two-item scales. I am guessing that it is impossible to
> > test for DIF.
> > Is that correct?
> > Any suggested reading on the minimum number of items required to test
> > for DIF in a Rasch model would be appreciated.
> >
> > Thanks,
> > Randy MacIntosh
> >
> > ________________________________________
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> > email: Rasch at acer.edu.au
> > web: https://mailinglist.acer.edu.au/mailman/options/rasch/svkr%
> 40sund.ku.dk
> > ________________________________________
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> trevor.bond%40jcu.edu.au
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