[Rasch] Fitting the RM

Anthony James luckyantonio2003 at yahoo.com
Tue Jul 8 01:30:42 EST 2008

Thomas and Trevor,
Thanks for your replies.
I can followe your argumentation.
however, the expressions I mentioned are very common even among Rasch practitioners. 
when one says "the Rasch model was fitted to test X", this cannot imply manipulation of the model to fit the data. because there is only one RM (irrespective of its family of models) and the researchers hasn't shifted to another model to account for the data at hand. my questions is a questions of Rasch measuremnt wording and terminology.
how about if we say "test X was fitted to the RM"?

--- On Mon, 7/7/08, Thomas Salzberger <Thomas.Salzberger at wu-wien.ac.at> wrote:

From: Thomas Salzberger <Thomas.Salzberger at wu-wien.ac.at>
Subject: Re: [Rasch] Fitting the RM
To: rasch at acer.edu.au
Date: Monday, July 7, 2008, 9:06 AM


I don't add anything new to what Trevor already said but just want to reinforce the idea.
Retrospectively, it was one of the most important insight I gained concering measurement from a Rasch point of view. 
We do not (only) choose the Rasch model because of its elegance or simplicity (relative to more complex IRT models) but because of its unique properties and its relationship to the principles of fundamental measurement. That's why the model enjoys a special status, it has priority over the data insofar as we are not willing to abandon or change the model if it happens to misfit data. Instead, we say that the data misfit the model, in other words the data don't have the structure that allows us to derive linear measures.

Personally, I would restrict the usage of the term "to fit" to statements like "data fit the model" or "data don't fit the model". I would rather not use it in the context of an activity. I would certainly not say "I fit the model to the data" as this implies changes to the model which are unjustified. Likewise "I fit the data to the model" may suggest changes to the data (although in some cases we do change the data in a sense, e.g. when rescoring items, but then this should not be the final conclusion).

In any case, I would distinguish between the Rasch model (with all its philosophical underpinnings) and an 1pl-model (without these theoretical foundations) in a more general IRT context. Most IRT people do appreciate the Rasch model's elegancy but they don't approve its special relationship to measurement. For them it is just convenient and if it misfits they switch to a more general model and fit that model to the data. (In most cases they don't even start with the Rasch model as they see it as overly restrictive.)

Best wishes

At 12:18 07.07.2008, Trevor Bond wrote:

Dear Anthony,
We fit data to the model: see B&F2, chap 12 for the detail.
Many others try to find models to fit the data in order to describe the data

we aim to have the data to fit the model to create measures...or to understand 'why not?'
At 2:57 AM -0700 7/7/08, Anthony James wrote:

Dear all,
I have come across expressions such as "fitting the Rasch model (or 2PL, 3PL) to Test X" or "the Rasch model fits (or does not fit)", "the Rasch model was fitted".
My question is when we "fit the Rasch model to a test" we are more emphasizing the model rather than the data. I was curious to know if these wordings have any special meaning and implications. Or do they simply mean Rasch analyzing the data and finding the misfitting items and persons and other routine analyses?

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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
Book: http://www.researchmethodsarena.com/books/Applying-the-Rasch-Model-isbn9780805854626 
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