[R-pkgs] package ltm -- version 0.9-0

Dimitris Rizopoulos d.rizopoulos at erasmusmc.nl
Mon Mar 2 19:11:27 CET 2009

Dear R-users,

I'd like to announce the release of the new version of package 'ltm' 
(i.e., ltm_0.9-0 soon available from CRAN) for Item Response Theory 
analyses. This package provides a flexible framework for analyzing 
dichotomous and polytomous data under various IRT models. Furthermore, 
supporting functions for descriptive statistics, goodness-of-fit, 
ability estimation and plotting are available.

New features include:

  * function gpcm() (along with supporting methods, for the generic 
functions, anova, plot, margins, factor.scores, summary, vcov) has been 
added for fitting the Generalized Partial Credit Model. The 'constraint' 
argument of gpcm() can be used to specify constrained versions of the 
model. Options are:

    - 'constraint = "rasch"' that fits the partial credit model (i.e., 
discrimination parameter fixed at one)

    - 'constraint = "1PL"' that specifies a single discrimination 
parameter, equal among items, and

    - 'constraint = "gpcm"' that specifies a different discrimination 
parameter per item.

  * gpcm() also allows for mixed type items, i.e., dichotomous and 
polytomous; however, for data sets with only dichotomous items, 
functions rasch() and ltm() are more efficient.

  * The new function GoF.gpcm() calculates the Pearson chi-squared 
statistic for Generalized Partial Credit models. A p-value can be also 
produced either using the asymptotic chi-squared distribution or a 
parametric Bootstrap approach.

  * The new function rmvordlogis() can be used to simulate ordinal data 
under the Graded Response and the Generalized Partial Credit models.

  * for more details and other new features check the NEWS file that 
ships with the package.

More information as well as *.R files illustrating the capabilities of 
the package can be found in the R-wiki page of 'ltm' available at: 

As always, any kind of feedback (e.g., questions, suggestions, 
bug-reports, etc.) is more than welcome.


Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014

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