ShinyItemAnalysis (version 1.5.4)

ggWrightMap: 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

Adela Hladka
Institute of Computer Science of the Czech Academy of Sciences

Jan Netik
Institute of Computer Science of the Czech Academy of Sciences

Patricia Martinkova
Institute of Computer Science of the Czech Academy of Sciences

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)
)