R: Profile Plots for IRT Models

profileplot {psychotools}

R Documentation

Profile Plots for IRT Models

Description

Base graphics plotting function for profile plot visualization of IRT models.

Usage

  profileplot(object, what = c("items", "thresholds", "discriminations"), 
    parg = list(type = NULL, ref = NULL, alias = TRUE),
    index = TRUE, names = TRUE, main = NULL, abbreviate = FALSE,
    ref = TRUE, col = "lightgray", border = "black", pch = NULL, cex = 1,
    refcol = "lightgray", linecol = "black", lty = 2, ylim = NULL,
    xlab = NULL, ylab = NULL, add = FALSE, srt = 45, adj = c(1.1, 1.1),
    axes = TRUE, ...)

Arguments

`object`	a fitted model object of class `"raschmodel"`, `"rsmodel"` or `"pcmodel"`.
`what`	character, specifying the type of parameters to be plotted.
`parg`	list of arguments passed over to internal calls of `itempar`, `threshpar` or `discrpar`.
`index`	logical, should different indexes for different items be used?
`names`	logical or character. If `TRUE`, the names of the items are displayed on the x-axis. If `FALSE`, numbers of items are shown. Alternatively a character vector of the same length as the number of items can be supplied.
`main`	character, specifying the overall title of the plot.
`abbreviate`	logical or numeric, specifying whether object names are to be abbreviated. If numeric this controls the length of the abbreviation.
`ref`	logical, whether to draw a horizontal line for the reference level. Only takes effect if argument `what` is `"items"` or `"discriminations".`
`col, border, pch, cex`	Graphical appearance of plotting symbols. Can be of the same length as the number of items, i.e., a different graphical appearance is used for each item. If `what = "thresholds"`, `col` and `pch` can be matrices with a number of columns equal to the number of threshold parameters per item resulting in different symbols and colors used for different threshold parameter profiles.
`refcol`	character, specifying the line color for the reference line (if `ref`).
`linecol`	character or numeric, specifying the line color to be used for the profiles.
`lty`	numeric, specifying the line type for the profiles.
`ylim`	numeric, specifying the y axis limits.
`xlab, ylab`	character, specifying the x and y axis labels.
`add`	logical. If `TRUE`, new plotted profiles are added to an existing plot.
`srt, adj`	numeric. Angle (`srt`) and adjustment (`adj`) in case names (rather than numbers) are used as x-axis labels. These are passed to `text`.
`axes`	logical. Should axes be drawn?
`...`	further arguments passed over to `plot`.

Details

The profile plot visualization illustrates profiles of specific estimated parameters under a certain IRT model.

Examples

## Load Verbal Aggression data
data("VerbalAggression", package = "psychotools")

## Fit Rasch, rating scale and partial credit
## model to VerbalAggression data
rmmod <- raschmodel(VerbalAggression$resp2)
rsmod <- rsmodel(VerbalAggression$resp)
pcmod <- pcmodel(VerbalAggression$resp)

## Profile plots of the item parameters of
## the three fitted IRT models
plot(rmmod, type = "profile", what = "items", col = 4)
plot(rsmod, type = "profile", what = "items", col = 2, add = TRUE)
plot(pcmod, type = "profile", what = "items", col = 3, add = TRUE)
legend(x = "topleft", legend = c("RM", "RSM", "PCM"),
  col = 1, bg = c(4, 2, 3), pch = 21, bty = "n")

## Profile plots of the threshold parameters of type 'mode'
plot(rmmod, type = "profile", what = "thresholds",
  parg = list(type = "mode"))
plot(rsmod, type = "profile", what = "thresholds",
  parg = list(type = "mode"))
plot(pcmod, type = "profile", what = "thresholds",
  parg = list(type = "mode"))

## Profile plot of the discrimination parameters
## of a dichotomous Rasch model (all zero by definition)
plot(rmmod, type = "profile", what = "discrimination")