irtoys-package: Estimate and plot IRT models for binary responses
In irtoys: A Collection of Functions Related to Item Response Theory (IRT)

irtoys-package

R Documentation

Estimate and plot IRT models for binary responses

Description

Package:	irtoys
Type:	Package
Version:	0.2.0
Date:	2016-01-30
License:	GPL (>= 2)
LazyLoad:	yes
LazyData:	yes

Details

The irtoys package contains a bunch of functions potentially useful to those teaching or learning Item Response Theory (IRT). R being particularly good at graphics, there is a rich array of plotting functions to visualize IRT models or assess their fit. Ability estimates can be estimated by MLE, BME, EAP, WLE. Various IRT scaling methods are supported: MM, MS, Stocking-Lord, and Hebaera. Last not least, irtoys may be the only package to estimate Haberman's interaction model, although a new and more powerful package is in the making.

Currently, there are several R packages that can estimate the item parameters in various uni- and multidimensional IRT models, but only several years ago one had to use stand-alone programs that had wildly different and often unwieldy user interfaces. Besides, no single program does everything one needs. One of the original purposes of irtoys was to provide a simple and unified interface to some of the most basic functions in ICL, BILOG, and R's own ltm, such that beginners could concentrate on learning IRT rather than syntaxes. Once that these steps have been made, those wishing to take advantage of the full functionality of ICL, BILOG & Co. must still master the syntax of their program of choice.

To take full advantage of irtoys, some IRT software is needed. Package ltm is automatically loaded. ICL by Brad Hanson can be downloaded from his site, http://www.openirt.com/b-a-h/software/. Executables and a very detailed manual (not needed to use ICL with irtoys are provided for Windows, Linux, and Macintosh. The Mac version probably does not work with the current OS versions but I still use the Windows and Linux versions without any problem.

BILOG is commercial software sold by SSI — see https://ssicentral.com/ for further detail.

On Windows, make sure that the executable files (icl.exe for ICL, BLM1.EXE, BLM2.EXE, and BLM3.EXE for BILOG) are located in a directory that is included in the PATH variable. On Linux, BILOG, being a Windows program, is run with wine, and should also be on a path where wine can find it. On my machine, I have simply put the three files in ~/.wine/drive_c/windows/. It seems that new versions of wine expect them to be explicitly tagged as executable. On Macintosh, at least ltm should work in all cases.

NOTE: Starting with version 0.2.0, function est returns a list of three elements: est contains the parameter estimates and is thus identical to the output in earlier versions, se contains the standard errors, in a similar format, and vcm contains the variance-covariance matrices (NULL when using ICL). When passing item parameters to another function that only needs the estimates, irf(ip) and irf(ip$est) can be used interchangeably. This facilitates using simulated item parameters. A function that does require the complete object is .

Also, function itf now returns item fit statistics as a vector rather than a list. Finally, since most of the functions in irtoys have been written with the "logistic" metric in mind (i.e., a_j(θ_i-b_j) rather than 1.7a^*_j(θ_i-b_j), function est now estimates item parameters only in the logistic metric.