ggWrightMap {ShinyItemAnalysis}

Plot person-item map (Wright map) using ggplot2

Description

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.

Usage

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.

Author(s)

Adela Hladka
Institute of Computer Science of the Czech Academy of Sciences
[e-mail]

Jan Netik
Institute of Computer Science of the Czech Academy of Sciences
[e-mail]

Patricia Martinkova
Institute of Computer Science of the Czech Academy of Sciences
[e-mail]

References

Wright, B. & Stone, M. (1979). Best test design. MESA Press: Chicago, IL

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)
)