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st: 2-PLM (was Rasch)
I completely agree with SR Millis : 2-PLM is not at all a Rasch model,
and generally is not a good solution when the Rasch model don't fit the
data. From a psychometrician point of view, 2-PLM (Birnbaum model) only
is a statictician model to obtain a good fit, but is not a
psychometrician model, because it have not good psychometrician
properties. Generally, it is recommanded to modify a scale if the Rasch
model don't fit it (for example, by reformulating or omitting
problematic items...), but it is not recommanded to use 2-PLM or more
complex model (3-PLM, 4-PLM, 5-PAM...).
Nevertheless, Zheng and Rabe-Hesketh recently have wrotten an excellent
paper in the Stata journal, which explain how to estimate the parameters
of the 2-PLM.
My own advice is to avoid this model, and to consider only the Rasch
model and its extensions : there is a very interesting extension of the
Rasch model wich is the OPLM (One Parameter Logistic Model) proposed by
Verhelst in the reference book of Fisher and Molenaar (1997): /Rasch
models. Foundations, recent developments and applications. / Springer
Verlag. With the OPLM, it is possible to adjust the discriminating
powers of the items whitout lost of the Rasch properties. This model can
be fit with gllamm.
Concerning the programing under Stata, -gllamm- is actually the more
important reference to estimate parameters of IRT models. Concerning the
Rasch model, -raschtest- allows estimating the parameters of the Rasch
model and evaluating the fit. Concerning the 2-PLM, there is no (to my
knowledge) specific Stata module, and the only solution is to use
-gllamm- which is in this case very slow, but other faster algorithm
have not be developped under Stata. Concerning the OPLM, it is possible
to estimate it with -gllamm- (marginal maximum likelihood - MML), or
with -mmsrm- (GEE algorithm, with several restrictions, notably that the
variance of the latent trait is lesser than 1 - this algorithm can, in
some cases, be faster than MML).
I hope this helps
Jean-Benoit Hardouin
Davood Souri a �crit :
You are right, I mean two-parameter item response model. I have tried
gllamm, but it is very slow and not suitable for large datasets. I am
wondering if there is another ado file in Stata.
Thanks
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of SR Millis
Sent: Wednesday, October 03, 2007 3:44 PM
To: [email protected]
Subject: Re: st: Rasch
There is no such thing as a 2-parameter Rasch model.
By definition, Rasch is a 1-parameter model.
SR Millis
--- Davood Souri <[email protected]> wrote:
Dear Statalisters,
Can anybody help me to estimate a two-parameter
Rasch model by Stata.
Thanks
Scott R Millis, PhD, MEd, ABPP (CN,CL,RP), CStat
Professor & Director of Research
Dept of Physical Medicine & Rehabilitation
Wayne State University School of Medicine
261 Mack Blvd
Detroit, MI 48201
Email: [email protected]
Tel: 313-993-8085
Fax: 313-966-7682
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--
Jean-Benoit Hardouin, PhD
Maitre de Conf�rences - Assistant Professeur
Team of Biostatistics, Clinical Research and Subjective Measures in Health Sciences
Departement of Biomathematics and Biostatistics
Faculty of Pharmaceutical Sciences
University of Nantes
1, rue Gaston Veil - BP 53508
44035 Nantes Cedex 1 - FRANCE
Email : [email protected]
Personal website : http://www.anaqol.org
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