[Rasch] Polytomous data difficulty parameter
trevor.bond at jcu.edu.au
Mon Sep 1 08:00:11 EST 2008
Which polytomous model did you use?
The RSM usually gives a difficulty estimate for each individual item plus one set
of threshold estimates common to all items.(ie problem solved)
The PCM usually gives four threshold estimates (for 5 cats) for each item.
Depending on the software you use, you can specify the item diff + threshold
ests as output from the PCM as well
c.f. Fig 6.3 (Quest) and Fig 6.4 Winsteps in B&F 2
BUT, why just the rank order of subscale items? Rasch measurement is designed
exactly for giving interval scales to mere orders
I am working with data from a 5-category questionnaire. I'd like to present a
table that indicates the rank order of subscale items by ease of endorseability.
>From what I have learned, polytomous data have difficulty parameters per
threshold, but not an overall item-level difficulty parameter. I have heard from
some that I should just a priori declare which threshold indicates endorsement,
and use that difficulty parameter. I have heard from others that I should average
all the threshold difficulties together to calculate an overall difficulty. First, is it
correct that there is no such thing as an overall difficulty parameter for
polytomous data (per item)? If no, how is it calculated. If yes, what is the best
approach to deriving a statistic that could meet my purpose mentioned above?
Gratefully yours, Mark
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