[Rasch] shift of person parameter scale

Mike Linacre mike at winsteps.com
Sat May 4 09:04:28 EST 2013


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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