Estimate and plot IRT models for binary responses

Description

Details

Provides a common interface to the estimation of item parameters in IRT models for binary responses with three different programs (ICL, BILOG, and ltm, and a variety of functions useful with IRT models.

The irtoys package contains a bunch of functions potentially useful to those teaching or learning Item Response Theory (IRT). Although there is no shortage of good IRT programs, those tend to have wildly different and often unwieldy user interfaces. Besides, no single program does everything one needs. Item parameters can be estimated with a program like ICL or BILOG, non-parametric approaches are implemented in TestGraf, transformation to a common scale needs ST, and so on. Some programs, such as ICL, have no graphical capabilities at all, while others offer stunning interactive graphics but refuse to output a Postscript file.

Package irtoys provides a common interface to some of the most basic functions in ICL, BILOG, and 's own ltm, some of the functionality of TestGraf and ST, and a variety of other functions. Those who want to take advantage of the full functionality of ICL, BILOG & Co. must still master their syntax.

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, www.b-a-h.com: executables are provided for Windows, Linux, and Macintosh. BILOG is commercial software sold by SSI — see www.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 0.1.6, function est returns a list of two matrices: est contains the parameter estimates and is thus identical to the output in earlier versions, and se contains the standard errors, in a similar format. 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.

Author(s)

Ivailo Partchev <partchev@gmail.com>

References

S. E. Embretson and S. P. Reise (2000), Item Response Theory for Psychologists, Lawrence Erlbaum Associates, Mahwah, NJ

Results