ggWrightMap() R function from [ShinyItemAnalysis]

Plot person-item map (Wright map) using `ggplot2`

This function allows to generate Wright map (also called person-item map) using ggplot() function from the ggplot2

package. Wright map is used to jointly display histogram of abilities (or other measured trait) and item difficulty parameters. Function takes pre-estimated parameter estimates, such as those obtained from an IRT model.


ggWrightMap(
  theta,
  b,
  binwidth = 0.5,
  color = "blue",
  size = 15,
  item.names,
  ylab.theta = "Respondent latent trait",
  ylab.b = "Item difficulty",
  rel_widths = c(1, 1)
)

Arguments

theta: numeric: vector of ability estimates.
b: numeric: vector of difficulty estimates.
binwidth: numeric: the width of the bins of histogram.
color: character: color of histogram.
size: text size in pts.
item.names: names of items to be displayed.
ylab.theta: character: description of y-axis for the histogram.
ylab.b: character: description of y-axis for the plot of difficulty estimates.
rel_widths: numeric: vector of length 2 specifying ratio of "facet's" widths.

Examples


library(mirt)

# fit Rasch model with the mirt package
fit <- mirt(HCI[, 1:20], model = 1, itemtype = "Rasch")
# factor scores
theta <- as.vector(fscores(fit))
# difficulty estimates using IRT parametrization
b <- coef(fit, simplify = TRUE, IRTpars = TRUE)$items[, "b"]

# Wright map
ggWrightMap(theta, b)

# Wright map with modified item names
item.names <- paste("Item", 1:20)
ggWrightMap(theta, b, item.names = item.names)

# Wright map with modified descriptions of y-axis and relative widths of plots
ggWrightMap(theta, b,
  ylab.theta = "Latent trait", ylab.b = "Difficulty estimates",
  rel_widths = c(2, 1)
)