[Rasch] shift of person parameter scale

Schulz, Matt mschulz at pacificmetrics.com
Sat May 4 09:52:57 EST 2013


I would wonder whether the desirable scale properties you are seeking with the transformation are worth the trouble it will be to apply the inverse transformation every time you need to (e.g., when you anchor items).  Wouldn't it be simpler to just apply the transformation to the person parameters?

From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On Behalf Of Rense Lange
Sent: Friday, May 03, 2013 4:49 PM
To: rasch at acer.edu.au
Subject: Re: [Rasch] shift of person parameter scale

To be sure: multiplications shouldn't be done to Rasch calibrated item parameters (adding - subtracting something is fine). Except, perhaps, to display them in the same metric as the (transformed) person measures.  But, don't multiply if you want to use the items later for anchoring, say.

Right?

Rense Lange

On May 3, 2013, at 6:04 PM, Mike Linacre <mike at winsteps.com<mailto:mike at winsteps.com>> wrote:


Thank you for your question, Felix.

You have a "Fahrenheit-to-Celsius" situation, equivalent to http://www.rasch.org/rmt/rmt223d.htm . You can subtract 0.5 from all the person and item location estimates, then multiply all the resulting estimates by 1.0/1.2. Also adjust discrimination parameters by 1.2/1.0.  Verify that these values are correct by computing some raw scores based on the rescaled estimates and comparing them with the original observed scores.

OK?

Mike L.

On 5/3/2013 7:02 AM, Fischer, Felix wrote:
I have been told that it is possible to shift item parameters of IRT models in order to achieve a certain distribution of person-estimates for a given sample. Practically I have a graded response model estimated in a larger sample including a representative sample. now, I want to shift the parameters that the distribution of the person estimates of the representative sample is normalized (has mean = 0 and sd = 1). Actually, the distribution is slightly different, eg mean = 0.5 and sd = 1.2.
Is anyone aware how to change the item-parameters? Or is there any reference where I could look up the procedure?
Best, Felix

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