PlotIRT: A Collection of R Functions to Plot Curves Associated With Item Response Theory

Abstract

The PlotIRT set of R functions facilitates plotting a variety of curves associated with item response theory (IRT). These functions were developed so that researchers using IRT could have a simple, yet flexible method for creating graphics associated with item parameters. These graphics can be customized in numerous ways to produce attractive figures that can be used in manuscripts or presentations. The R software environment was chosen so that users could produce IRT graphics via free software. The plots that are available in PlotIRT are item and test characteristic curves, item and test information curves, expected item score functions, and posterior density distributions. The R functions are applicable to the two-parameter logistic, three-parameter logistic, graded, nominal, and multiple-choice models. The functions work with both individual items or with groups of items, which do not need to be based on the same IRT model. If the user chooses, the graphics can be customized with a number of options. These options include varying the color of the lines by item, response category, or test; making the lines a solid, dashed, or other pattern; labeling the response category curves; specifying the number of items per plot and the number of plots per window; labeling the plot and axes; and adjusting the font and its size. The posterior distribution plot requires the user to specify a response pattern for the items, and it may be produced along with the item characteristic curves and the underlying y distribution. Basic R functions can be used to read in an assortment of item parameter files, or item parameters can be typed directly into R arrays. As the function is submitted, plots appear in windows within the R interface. An option is included in the function to save the plots as any file type supported by R, including postscript and portable document format.