ggWrightMap: Plot Wright map using 'ggplot2'

In netique/ShinyItemAnalysis: Test and Item Analysis via Shiny

Description

This function allows to generate Wright map (also called item-person map) using ggplot() function from the ggplot2 package. Wright map is used to display histogram of factor scores and the item difficulty parameters estimated by the Rasch 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)

data(HCI)

# 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)
)

netique/ShinyItemAnalysis documentation built on Dec. 22, 2021, 12:10 a.m.