# [Rasch] Unidimensionality

Steven L. Kramer skramer1958 at verizon.net
Thu May 8 06:41:46 EST 2008

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I too am trying to understand how unidimensionality looks in Rasch. Michael, I want to make sure I understand you correctly.

Lets say I have some easier multidigit whole number multiplication problems (no calculator permitted)

A. 28 * 91

some harder ones

B. 374*68

and some very hard ones

C. 2,986 * 77,864

Similarly, I have easy, hard and very hard long division

D. 527/6

E. 3,588/21

F. 4,877,864/876

Are you saying that a factor analysis might find two factors (multiplication and division) whereas Rasch might find one?  If so, I'd have to question the Rasch answer.
Or, am I misunderstanding?

Steve Kramer
----- Original Message -----
From: Michael Lamport Commons
To: luckyantonio2003 at yahoo.com ; rasch at acer.edu.au
Sent: Wednesday, May 07, 2008 4:13 PM
Subject: Re: [Rasch] Unidimensionality

Life is more complected.  Let us say one is scaling difficulty of tasks.  One generally will get a single dimension in Rasch but 2 or more dimensions in a factor analysis if ... difficulty is made up of order of hierarchical complexity and also the number of operations such as the number of numbers added.  Difficulty can be seen as one dimension or two depending on how much you know what the more gross variable consists of.

Michael Lamport Commons

-----Original Message-----
From: Anthony James
Sent: May 7, 2008 12:54 PM
To: rasch at acer.edu.au
Subject: [Rasch] Unidimensionality

Dear colleagues

In a couple of papers I noticed that the researchers before using Rasch model or IRT models first use factor analysis to ascertain unidimensionality. Since unidimensionality is a prerequisite to use IRT models. They give the impression that only after FA has shown that the test is unidimensioanl, one can use Rasch or IRT models.

Is this really necessary? Isn’t the Rasch model itself a technique, superior to FA, to demonstrate unidimensionality?

Cheers

Anthony

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Michael Lamport Commons, Ph.D.

Assistant Clinical Professor
Department of Psychiatry
Harvard Medical School

Program in Psychiatry and the Law
Beth Israel Deaconess Medical Center
commons at tiac.net

http://www.dareassociation.org/
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