[Rasch] DIF studies for polytomous items
yk0271 at yahoo.co.jp
Sat Apr 17 00:52:12 EST 2010
Thank you all very much for your supports for my previous question regarding DIF studies for polychromous items. Today, I would like to ask another question that is partly related to the previous question that I asked.
The question is about which threshold values--Rasch-Andrich threshold or Rasch-Thurston threshold--I should use to investigate DIF of polytomous items.
My understanding of a DIF analysis process is like this. Step 1, given that there are two groups—-reference and focal groups--for DIF investigation, I need to combine the data of these groups as one to estimate a set of category threshold values. The step 2 will be that I use this set
of threshold values as anchors to recalibrate item difficulties for the two groups separately (and so on…).
My question is regarding the step 2. Which threshold values--Rasch-Andrich threshold or Rasch-Thurston threshold—I should use? I use FACETS program to conduct this investigation based on the rating scale model. In my situation, I think I need to use the Rasch-Thurston threshold values,
but it seems that I am not able to use them as anchor values.
So, my question is about the following two issues:
(1) Is it correct to use the Rasch-Thurston threshold values as anchors to make item difficulties comparable across two groups in a DIF investigation?
(2) If the first question is true, does FACETS have a capability to make Rasch-Thurston threshold values as anchors?
Thank you very much.
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