[Rasch] DIF studies for polytomous items

Kenji Yamazaki yk0271 at yahoo.co.jp
Sat Apr 17 00:52:12 EST 2010

Dear colleagues,
  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.
  Kenji Yamazaki

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