[Rasch] Invariance

Rama Guinda guinda36 at hotmail.com
Thu Jan 5 23:20:45 EST 2006


Dear All,
I have a question. Thanks for any reaction to my problem.
I Rasch analysed 4 polytomous items with the Winsteps. The items where 4 
texts each with 20 gaps to be filled by the test-takers. In order to avoid 
the problem of local dependence I considered each text a super item. I had 
160 cases.
The item measures are .74, .06, -.29 and -.50.
Then I divided the group into two (a high ability and a low ability group) 
on the basis of a  reading test which contains 14 dichotomously scored 
items. Students with scores from 0 to 7 went into the low ability group and 
the ones with scores from 8 to 14 went into the high ability group. The 
reading test has a significant but low correlation with the gap fill task 
(.53).
I had 98 cases in the low ability group and 62 cases in the high ability 
group. To study the item invariance I separately analysed the data for each 
group.
The item measures for the low ability group are: .62, .10, -.25, -.47.
The item measures for the high ability group are: .94, .02, -.38, -.58.
The order of the difficulty of the items remains the same in all three 
groups.
Question 1: What is the test of invariance for the item measures? Have they 
really remained invariant within the allowable range of error?

Best
Rama

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