[Rasch] many facet Rasch model

rsmith.arm at att.net rsmith.arm at att.net
Fri Mar 10 09:30:52 EST 2006


My random groups consisted of 15,000 people each, across 13 test forms, for a total of 180,000 people and still there was a difference of .5 logits in the group means.  Until you can demonstrate empirically that the groups are equivalent, it is a very tenuous assumption.
--
Richard M. Smith 
12276 Arbor Lakes Parkway North 
Maple Grove, MN 55369 
voice(w): 763-268-2282 
voice(h): 763-494-5047 






-------------- Original message from Ryan Patrick Bowles <rpb3b at cms.mail.virginia.edu>: -------------- 


> Barth's suggestion was quite reasonable and a good idea 
> when rater overlap is not possible. Randomly equivalent is 
> a perfectly justifiable statistical assumption. It is the 
> basis of random assignment in clinical trials: the control 
> and treatment groups must be randomly equivalent. In 
> reality, they never are perfectly equivalent, but provided 
> the sample is large enough, the groups will be close enough 
> to equivalent that differences are negligible. 
> 
> For this example, linking different rater groups requires 
> only an assumption of equal means, with no assumptions 
> about higher moments. This is a relatively weak 
> requirement. Therefore, random equivalence is sufficient to 
> yield good linking with relatively small group sizes. Five, 
> however, is not a large enough group size to make an 
> assumption of equivalence as a result of random assignment 
> reasonable. 
> 
> The comparison with test books is not entirely appropriate. 
> There are two sources of sampling error: assigning persons 
> to test books; and assigning items to test books. Noting 
> large differences cannot be attributed to failure of random 
> equivalence without further information. 
> 
> Ryan 
> 
> --On Thursday, March 09, 2006 9:17 PM +0000 
> rsmith.arm at att.net wrote: 
> 
> > 
> > 
> > Interesting idea, Randomly Equivalent! When I learned 
> > statistics random assignment ment random not randomly 
> > equivalent. In state testing programs, where test books 
> > are randomly assigned, I have seem mean ability 
> > differences as high as 0.5 logits across forms. Hardly 
> > equivalent. Any equating based on the idea that the 
> > groups were randomly equivalent is doomed to failure. 
> > -- 
> > Richard M. Smith 
> > 12276 Arbor Lakes Parkway North 
> > Maple Grove, MN 55369 
> > voice(w): 763-268-2282 
> > voice(h): 763-494-5047 
> > 
> > 
> > 
> > 
> > 
> > -------------- Original message from "Barth Riley" 
> > : -------------- 
> > 
> > 
> > 
> > Hi Susan 
> > 
> > 
> > 
> > What will most likely happen is that Facets will be 
> > unable to link ratings of exhibitions across panels. 
> > Therefore, it will not be possible to compare an exhibit 
> > rated by panel #1 to an exhibit rated by panel #2, etc. 
> > If data collection is still ongoing, I would strongly 
> > encourage moving raters to multiple panels to ensure a 
> > linkage across panels. The alternative strategy is to 
> > randomly assign raters to panels. Then we would assume 
> > that the panels are “randomly equivalent” and then anchor 
> > each panel to a common logit value, typically 0. ‘This 
> > group anchoring method can sometimes, though not always, 
> > allow disjointed subsets in the data to be connected by 
> > Facets. Generally, the more overlap, the better. 
> > 
> > 
> > 
> > Barth 
> > 
> > 
> > 
> > 
> > 
> > Barth Riley, Ph.D. 
> > 
> > Res. Asst. Professor & Associate Program Director 
> > 
> > Dept. of Disability and Human Development M/C 626 
> > 
> > University of Illinois-Chicago 
> > 
> > 1640 W. Roosevelt Rd. 
> > 
> > Chicago, IL 60608 
> > 
> > Voice (312) 355-4054 
> > 
> > Fax: (312) 355-4058 
> > 
> > Email: barthr at uic.edu 
> > 
> 
> 
> 
> Ryan Bowles 
> Department of Psychology 
> University of Virginia 
> P.O. Box 400871 
> Charlottesville, VA 22904-4871 
> 434-982-6508 
> rpbowles at virginia.edu 
> 
> _______________________________________________ 
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