[Rasch] OPLM

Stephen.Humphry at det.wa.edu.au Stephen.Humphry at det.wa.edu.au
Wed Jul 5 17:14:24 EST 2006

Rama, the OPLM is formally the same as the 2PL model. However, item
discrimination in the OPLM is treated as an index rather than a parameter.
The index is imputed rather than being estimated. So the 'one' in OPLM
implies there is still only one item parameter.

The weighted score in the 2PL model is sufficient for the person parameter
but the problem is that the values of discrimination parameters are unknown,
meaning the weighted raw score "is not a mere statistic, and hence it is
impossible to use CML as an estimation method" (Verhelst & Glas, 1995, p.
217). The approach in the OPLM is to impute the values rather than
estimating them, though estimates are still used as starting point. The
index is restricted to integer values between 1 and about 15.

The OPLM is intended to retain the properties of the Rasch model while
giving the 'flexibility' of the 2PL model (stated at the beginning of the


Stephen Humphry, PhD
Senior Educational Measurement Officer, Psychometrics
Department of Education & Training
151 Royal Street, East Perth, 6004
Phone: +61 8 92644102

Verhelst, N.D. & Glas, C.A.A. (1995).  The One Parameter Logistic Model.  In
G.H. Fischer and I.W. Molenaar (Eds.), Rasch Models: Foundations, recent
developments and applications (pp. 215-237).  New York: Springer-Verlag

-----Original Message-----
From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On Behalf
Of Rama Guinda
Sent: Wednesday, 5 July 2006 2:31 PM
To: rasch at acer.edu.au
Subject: [Rasch] OPLM

Dear All,
Could you please answer this question.
What is OPLM?
I know that it stands for one parameter logistic model and is also the
name of a Rasch programme written by  Norman Verhelst in the
Is it basically the simple Rasch model or a special extension of  it?
How is it different from the Rasch model?

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