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\name{infoplot}
\alias{infoplot}
\title{Information Plots for IRT Models}
\description{
Base graphics plotting function for information plot visualization of IRT models.
}
\usage{
infoplot(object, what = c("categories", "items", "test"),
ref = NULL, items = NULL, names = NULL, layout = NULL, xlim = NULL,
ylim = NULL, col = NULL, lty = NULL, lwd = NULL, main = NULL, legend = TRUE,
xlab = "Latent trait", ylab = "Information", add = FALSE, \dots)
}
\arguments{
\item{object}{a fitted model object of class \code{"raschmodel"},
\code{"rsmodel"}, \code{"pcmodel"}, \code{"plmodel"} or \code{"gpcmodel"}.}
\item{what}{character, specifying the type of information to visualize.}
\item{ref}{argument passed over to internal calls of \code{\link{predict}}.}
\item{items}{character or numeric, specifying the items for which information
curves should be visualized.}
\item{names}{character, specifying labels for the items.}
\item{layout}{matrix, specifying how the item or category information curves
of different items should be arranged. If \code{null} and \code{what} is set
to \code{"items"}, the item information curves are overlayed within a single
plot.}
\item{xlim, ylim}{numeric, specifying the x and y axis limits.}
\item{col}{character, specifying the colors of the test, item or category
information curves.}
\item{lty}{numeric, specifying the line type of the information curves.}
\item{lwd}{numeric, specifying the line width of the information curves.}
\item{main}{character, specifying the overall title of the plot.}
\item{legend}{logical, specifying if a legend is drawn when multiple item
information curves are overlayed. The labels in the legend correspond to
the item names (which can be specified in the argument \code{names}).}
\item{xlab, ylab}{character, specifying the x and y axis labels.}
\item{add}{logical. If \code{TRUE}, new information curves are added to
an existing plot. Only possible for a test or a single item information
curve.}
\item{\dots}{further arguments passed to internal calls of
\code{\link{matplot}}.}
}
\details{
The information plot visualization illustrates the test, item or category
information as a function of the ability parameter \eqn{\theta} under a
certain IRT model. Further details on the computation of the displayed
information can be found on the help page of the function
\code{\link{predict.pcmodel}}.
}
\seealso{\code{\link{curveplot}}, \code{\link{regionplot}},
\code{\link{profileplot}}, \code{\link{piplot}}}
\examples{
## load verbal aggression data
data("VerbalAggression", package = "psychotools")
## fit Rasch and partial credit model to verbal aggression data
rmmod <- raschmodel(VerbalAggression$resp2)
pcmod <- pcmodel(VerbalAggression$resp)
## category information plots for all items under the dichotomous RM
plot(rmmod, type = "information", what = "categories")
## category information plots for all items under the PCM
plot(pcmod, type = "information", what = "categories")
## overlayed item information plots for the first six items of the
## data set under the PCM
plot(pcmod, type = "information", what = "items", items = 1:6)
## a comparison of the item information for the first six items under the
## dichotomous RM and the PCM
plot(pcmod, type = "information", what = "items", items = 1:6,
xlim = c(-5, 5))
plot(rmmod, type = "information", what = "items", items = 1:6,
lty = 2, add = TRUE)
legend(x = "topright", legend = c("PCM", "RM"), lty = 1:2, bty = "n")
## a comparison of the test information based on all items of the
## data set under the dichotomous RM and the PCM
plot(pcmod, type = "information", what = "test", items = 1:6, xlim = c(-5, 5))
plot(rmmod, type = "information", what = "test", items = 1:6, lty = 2,
add = TRUE)
legend(x = "topright", legend = c("PCM", "RM"), lty = 1:2, bty = "n")
if(requireNamespace("mirt")) {
## fit 2PL to verbal aggression data
twoplmod <- plmodel(VerbalAggression$resp2)
## category information plots for all items under the dichotomous 2PL
plot(twoplmod, type = "information", what = "categories")
}
}
\keyword{aplot}
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