# [Rasch] many facet Rasch model

Michael Lamport Commons commons at tiac.net
Fri Mar 10 09:39:02 EST 2006

```Is what one should do is to give 2 or more forms in random order to the same participants?
My Best,

Michael Lamport Commons, Ph.D.
Assistant Clinical Professor
Program in Psychiatry and the Law
Department of Psychiatry
Harvard Medical School
Beth Israel Deaconess Medical Center
234 Huron Avenue
Cambridge, MA 02138-1328

Telephone (617) 497-5270
Facsimile (617) 491-5270
Commons at tiac.net
http://dareassociation.org/

----- Original Message -----
From: rsmith.arm at att.net
To: Ryan Patrick Bowles ; Rasch Listserve
Sent: Thursday, March 09, 2006 5:30 PM
Subject: RE: [Rasch] many facet Rasch model

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
> > differen ces 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 ongoin g, 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. Roose velt 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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