[Rasch] Help with equating partial credit items in pre - post test

Andrés Burga León 21781 at upch.edu.pe
Sun Jun 1 09:39:31 EST 2008

Hello to everybody on the list.


I have some doubts about how to equate pre 7 post test results in a math
test. I want to put the two measures on the post test frame of reference.


The problem is that most of the items are politomous, scored on a 0 to 2
scale, and the threshold are different for the pre / post test calibrations.


On the pre test, most items show disordered Rasch – Andrich thresholds (most
responses are 0 score, only few 1 or 2). In the post test, the situation for
some items is contrary. There are also disordered thresholds, but the
answers are concentrated on the 2 score).


Other items, that showed disordered thresholds on the pre – test know show
ordered Rasch- Andrich thresholds. I think that I should only use one of the
thresholds in order to anchor the pre-test results, but I’m not sure about
the effects of the differences in threshold structures on the validity of
the measures


Which could be the best way to put pre test results on the post – test frame
of reference?


Thank you very much for your advise



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