piplot {psychotools}

Person-Item Plots for IRT Models

Description

Base graphics plotting function for person-item plot visualization of IRT models.

Usage

  piplot(object, pcol = NULL, histogram = TRUE, ref = NULL, items = NULL,
    xlim = NULL, names = NULL, labels = TRUE, main = "Person-Item Plot",
    xlab = "Latent trait", abbreviate = FALSE, cex.axis = 0.8, cex.text = 0.5,
    cex.points = 1.5, grid = TRUE, ...)

Arguments

object

a fitted model object of class "raschmodel", "rsmodel", "pcmodel", "nplmodel" or "gpcmodel".

pcol

optional character (vector), specifying the color(s) used for the person parameter plot.

histogram

logical. For models estimated via MML (nplmodels and gpcmodels), should a histogram of the person-wise (individual) person parameters be drawn additionally to the normal distribution density of the person parameters?

ref

argument passed over to internal calls of threshpar and itempar.

items

character or numeric, specifying the items which should be visualized in the person-item plot.

xlim

numeric, specifying the x axis limits.

names

character, specifying labels for the items.

labels

logical, whether to draw the number of the threshold as text below the threshold.

main

character, specifying the overall title of the plot.

xlab

character, specifying the x axis labels.

abbreviate

logical or numeric, specifying whether object names are to be abbreviated. If numeric, this controls the length of the abbreviation.

cex.axis

numeric, the magnification to be used for the axis notation relative to the current setting of cex.

cex.text

numeric, the magnification to be used for the symbols relative to the current setting of cex.

cex.points

numeric, the magnification to be used for the points relative to the current setting of cex.

grid

logical or color specification of horizontal grid lines. If set to FALSE or "transparent" grid lines can be suppressed.

...

further arguments passed to internal calls of lines, points and text.

Details

The person-item plot visualization illustrates the distribution of the person parameters against the absolute item threshold parameters under a certain data set and IRT model. For models estimated via MML (nplmodels and gpcmodels), the normal distribution density of the person parameters is drawn. If histogram is set to TRUE (the default), a histogram of the person-wise (individual) person parameters is drawn additionally. If a multiple group model has been fitted by supplying an impact variable, multiple person parameter plots are drawn, each corresponding to a specific level of this variable.

See Also

curveplot, regionplot, profileplot, infoplot

Examples

## load verbal agression data
data("VerbalAggression", package = "psychotools")

## fit partial credit model to verbal aggression data
pcmod <- pcmodel(VerbalAggression$resp)

## create a person-item plot visualization of the fitted PCM
plot(pcmod, type = "piplot")

## just visualize the first six items and the person parameter plot
plot(pcmod, type = "piplot", items = 1:6, pcol = "lightblue")


if(requireNamespace("mirt")) {
## fit generalized partial credit model to verbal aggression data
gpcmod <- gpcmodel(VerbalAggression$resp)

## create a person-item plot visualization of the fitted GPCM
plot(gpcmod, type = "piplot")

## turn off the histogram and grid
plot(gpcmod, type = "piplot", histogram = FALSE, grid = FALSE)

## fit GPCM to verbal aggression data accounting for gender impact
mgpcmod <- gpcmodel(VerbalAggression$resp, impact = VerbalAggression$gender)

## create a person-item plot visualization of the fitted GPCM
plot(mgpcmod, type = "piplot", pcol = c("darkgreen", "darkorange"))
}