[Rasch] FW: Why isn't the person separation better?

Stephanou, Andrew Stephanou at acer.edu.au
Wed Sep 1 08:55:36 EST 2010


Forwarded to the Rasch listserv on behalf of Trudy Mallinson,
trudy.mallinson at usc.edu


-----Original Message-----
From: mallinso at usc.edu [mailto:mallinso at usc.edu] On Behalf Of Trudy
Mallinson
Sent: Wednesday, 1 September 2010 3:00 AM
To: rasch
Subject: Why isn't the person separation better?

Dear Listserv,

I am analyzing a small data set (about 200 participants, both admission
and discharge currently in the dataset) and 32 items.  Everything about
the instrument looks good - the items mostly fit, the rating scale (3
points) mostly works. The range of persons seems to form a fairly normal
distribution, items are targeted on the people (mean -0.28 logits) with
good range (min -4.07 logits, max 3.55 logits).  But the person
separation is less than stellar at .84.  There are a few items where the
middle category of the rating scale is "submerged" but not disordered.
When I combine it with one of the other categories, the SD increases a
little but so does the error so it's a wash in terms of the separation.
I tried removing the misfitting items (not very misfitting at 1.3 - 1.4
infit MNSQ) same thing, increased SD, increased error, no change in
separation.  I wondered if it could be a dimensionality problem - the
PCA suggests it's unidimensional though.  I separated out the items into
two groups.  Item psychometrics continue to look OK.  Separation for the
vision items .82, for tactile items .82.  I created an xy plot of the
person measures from the two set so items.  Scores clearly track
together although the shape of the coordinates is basically a
parallelogram.
Can any one suggest why, when everything about the items looks good, the
separation is not great, and all the things I've tried don't seem to
improve it in any meaningful way?

Thanks for your thoughts,
Trudy

Trudy Mallinson, PhD, OTR/L, NZROT
Assistant Professor
University of Southern California
Division of Occupational Science and Occupational Therapy
1540 Alcazar Street, CHP 133, Room 101F
Los Angeles, CA 90089-9003
PH: (323) 442-2950
trudy.mallinson at usc.edu
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