[Rasch] Misfitting Individuals

Twing, Jon jon.s.twing at pearson.com
Sun Apr 29 12:24:59 EST 2007



This is often more art than science.  Here is what we sometimes do:


1.)     Use Rasch Person Fit to identify "anomalies" in the testing
experience (this could be pure guessing, cheating or other unusual
student engagements).

2.)     Since most of our work requires a student score, we will score
them but we might choose to drop them from the calibration.

3.)     In the "old days" we might have included an asterisk indicating
peculiar response string, but we have not done this in the last 10 years
or so.

4.)     Typically we use Mean Square Fit, INFIT and OUTFIT when
diagnosing person anomalies.  We typically use the values dictated for
items and apply them to persons.

5.)     Below are the criteria I have collected over the years.


Item INFIT and OUTFIT should be between 0.60 and 1.40 (Bond & Fox, 2001;
Linacre & Wright, 1999)

Item INFIT between 0.70 and 1.30 (Bode, Heineman, & Semik, 2000; Bogner,
Corrigan, Bode & Heinemann, 2000).

Mean-square fit statistics are defined such that the model-specified
uniform value of randomness is 1.0.  Values greater than 1.5 (more than
50% unexplained randomness) are problematic. (Wright and Panchapakesan,
1969; Linacre, 1999).


Hope this helps.  Good luck.





Jon S. Twing, Ph.D.

Executive Vice President, Test & Measurement Services


Pearson Educational Measurement

2510 N. Dodge Street, P.O. Box 30, Mailstop 165

Iowa City, Iowa  52245-9945

Phone: 319-339-6407

Fax: 319-339-6477

Cell: 319-331-6547


Jon.S.Twing at Pearson.com





From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On
Behalf Of Petroski, Greg
Sent: Friday, April 27, 2007 1:29 PM
To: rasch at acer.edu.au
Subject: [Rasch] Misfitting Individuals


I have a few questions centering on person-fit.  The need to understand
the cause of aberrant response patters is obvious. But in applications,
i.e. not in the test development phase, what is does with misfitting


Score them anyway?

Exclude them from the reporting?  This could be a very unpopular
solution in some applications.  


Are rules of thumb for person INFIT and OUTFIT the same as when judging
item fit?  In which case one might not report scores for individuals
with INFIT or OUTFIT exceeding certain limits.  Is this done?



Gregory F. Petroski, Ph.D.

Dept. of Health Management and Informatics &
Office of Medical Research/Biostatistics
137 Hadley Hall (DC 018)
University of Missouri - Columbia,
Columbia, Mo.  65212





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