[Rasch] Rasch and Missing data
ici_kalt at yahoo.com
Sat Jul 5 03:23:47 EST 2008
Thank you Thomas for the precise explanation.
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--- On Fri, 7/4/08, Thomas Salzberger <Thomas.Salzberger at wu-wien.ac.at> wrote:
From: Thomas Salzberger <Thomas.Salzberger at wu-wien.ac.at>
Subject: Re: [Rasch] Rasch and Missing data
To: rasch at acer.edu.au
Date: Friday, July 4, 2008, 12:09 PM
At 18:58 04.07.2008, Agustin Tristan wrote:
" the score out of the three easy items can't be compared to the score on the three hardest items. You have to convert raw scores to linear measures first to derive comparable measures."
The explanation concerning raw score and true measures is very difficult to maintain when you see the results of any Rasch analysis program if there are no missing answers. If you have 3 correct answers in a test of 30 dichotomous items, even if you have the 3/30 hardest and me the 3/30 most easy, you and me will have the same measure (same raw score by the way), but we shall have different fit. If I see the output of a Rasch analysis, you will have 3/30 as me, you will have a measure say -2.0 as me! Measure alone will not reflect the difference of items we did answer.
3/30 leads to the same measures if we both answered all items (and the same items) (but, of course, fit differs considerably).
I referred to a situation where we DID NOT respond to the same items. I answered the three easiest ones (but the three hardest ones were not presented to me or whatever the reason for missingness may be), you answered the three hardest ones, so we cannot compare raw scores.
The entire paragraph reads (emphasis added):
Even with the same max score you may not be able to compare raw scores. If you have two persons with three missings each (one person misses the three easiest items, the other person misses the three hardest), they both have a theoretical score range of 0 to 15 but still the score out of the three easy items can't be compared to the score on the three hardest items. You have to convert raw scores to linear measures first to derive comparable measures.
Maybe the confussion arises from my wording "the person misses the three hardest items". I meant that the person has missing values there and not a score of zero.
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