Dear Colleagues:

     I am happy to announce that the GGUM2000 software 
system is now available free of charge.  The GGUM2000 
system estimates parameters for a family of item response 
theory models for unfolding.  The most general model 
implemented in the system is the generalized graded 
unfolding model (GGUM) that was described in the March 
2000 issue of Applied Psychological Measurement (pp. 3-
32).  In addition to this very general model, the 
GGUM2000 system also estimates seven other models that 
can be obtained by constraining item parameters from the 
GGUM in alternative ways.  The system estimates item 
parameters using marginal maximum likelihood, and 
person parameters are estimated using an expected a 
posteriori (EAP) technique.  The program allows for binary 
or polytomous responses, up to 100 items with 2-10 
response categories, and up to 2000 respondents.  The 
GGUM2000 system is a DOS-based program and is 
accompanied by an informative user's manual in 
WordPerfect 6.1 for Windows format.  The program can be 
downloaded from a web site devoted to IRT models for 
unfolding.  The site is located at:

http://www.education.umd.edu/EDMS/tutorials/index.html

To obtain the software, click on the "Free Software to 
Construct IRT Unfolding Models" and you will be taken to 
the GGUM2000 advertisement.  Click on "Download  
GUMIT2.EXE", then do the same thing on the next screen 
that appears. 

     The GGUM2000 system is supported by the author.  
Your feedback is appreciated and will be used to improve 
subsequent versions of the system.  

     While you are at the web site, please notice the other 
features available to you.  There is an extensive reference 
page that provides a current list of books and articles on 
IRT-based approaches to unfolding.  There is also an 
example data sets page from which illustrative test data 
may be downloaded.  Finally, there is a listing of 
commercially available IRT-based unfolding software.

     I hope you will stop by the web site soon and get your 
free copy of GGUM2000.   For those readers who may not 
be familiar with IRT models for unfolding, I have included 
a clip from the user's manual below.  Although it has 
increased the length of this post substantially, I hope some 
folks find it useful.

Best Wishes,
Jim Roberts



                    What is GGUM2000?

     The GGUM2000 system is a software package that 
estimates parameters from a family of item response theory 
(IRT) models known as "unfolding models".  These models 
assume that persons and items can be jointly represented as 
locations on a latent unidimensional continuum.  A single-
peaked, nonmonotonic response function is the key feature 
that distinguishes unfolding IRT models from traditional, 
"cumulative" IRT models.  This response function suggests 
that a higher item score is more likely to the extent that an 
individual is located close to a given item on the underlying 
continuum.  In contrast, cumulative IRT models imply that 
a higher item score is more likely when the location of the 
individual exceeds that for the item on the latent 
continuum.

     The unfolding IRT models implemented in the 
GGUM2000 system are appropriate for measuring a variety 
of constructs.  For example, the models are well suited to 
measure individual attitudes using data from either 
Thurstone or Likert attitude questionnaires (Andrich, 1996; 
Roberts, 1995; Roberts, Laughlin & Wedell, 1999).  With 
these questionnaires, respondents indicate how much they 
disagree or agree with each statement.  The response may 
be binary (0=disagree, 1=agree) or graded (0=strongly 
disagree, 1=disagree, 2=slightly disagree, 3=slightly agree, 
4=agree, 5=strongly agree), but in each case, higher levels 
of agreement are coded with successive integers.  In the 
context of attitude measurement, these unfolding models 
predict more agreement to the extent that an individual's 
opinion is similar to the sentiment expressed by the item.  
The individual's location on the continuum is a measure of 
the individual's attitude and the item's location is a 
measure of its sentiment (i.e., its scale value).  

     These unfolding models are also relevant to preference 
measurement situations where a respondent indicates how 
much he/she prefers each stimulus in a set of I stimuli.  
Suppose preference judgments are obtained from a sample 
of respondents using a rating scale with 0 to C scale points 
where a response of C represents the highest degree of 
preference.  In this situation, one might postulate that 
respondents and stimuli are jointly located on a 
unidimensional continuum.  The location of a given 
respondent represents the respondent's "ideal point".  A 
respondent is expected to prefer a stimulus to the extent 
that it is located close to this ideal point.

     Finally, the unfolding models implemented in the 
GGUM2000 system can be used to measure developmental 
processes that occur in stages (Noel, 1999).  For example, 
individuals who attempt to quit smoking may proceed 
through a series of cognitive/behavioral stages ranging 
from precontemplation about the dangers of smoking, to 
contemplation about smoking hazards and the need to quit, 
to actively resisting the urge to smoke.  Statements 
representing each of these stages could be presented on a 
questionnaire, and respondents could be asked to indicate 
their level of agreement with each statement.  An unfolding 
model could be used to order the statements along a 
unidimensional developmental continuum that illustrates 
these stages of change.  Additionally, the model could also 
estimate the locations of respondents on this continuum.       



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James S. Roberts, Ph.D.
Assistant Professor
Department of Measurement, Statistics & Evaluation
University of Maryland
College Park, MD  20742
(301) 405-3630 Voice
(301) 314-9245 Fax
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