piplot

From psychotools v0.5-1 by Achim Zeileis

Person-Item Plots for IRT Models

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

Keywords: aplot

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", "plmodel" or "gpcmodel".
pcol: optional character (vector), specifying the color(s) used for the person parameter plot.
histogram: logical. For models estimated via MML (plmodels 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 (plmodels 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.

Examples

# NOT RUN {
## 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")

# }
# NOT RUN {
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"))
}
# }

Documentation reproduced from package psychotools, version 0.5-1, License: GPL-2 | GPL-3

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