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

Tom Bramley Bramley.T at cambridgeassessment.org.uk
Wed Sep 1 17:58:53 EST 2010

Dear Trudy,
Perhaps the problem is simply that there is not enough 'true' variance in your sample of participants.  What is the range and standard deviation of the raw scores (which I assume are on a scale of 0-64)?

Tom Bramley
Assistant Director, Research Division
Assessment Research & Development

Cambridge Assessment
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From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On Behalf Of Stephanou, Andrew
Sent: 31 August 2010 23:56
To: Rasch at acer.edu.au
Subject: [Rasch] FW: Why isn't the person separation better?

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 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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