[Rasch] shift of person parameter scale

Tom Conner connert at msu.edu
Sat May 4 10:08:48 EST 2013


Doesn't linear transformation involve both multiplication and 
addition/substraction?

On 5/3/13 7:49 PM, Rense Lange wrote:
> 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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>
>
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-- 
Tom Conner
Professor Emeritus
Department of Sociology
Michigan State University
"What if there were no hypothetical questions?"

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