Plot Item and Test Information Functions

This method plots item or test information functions for a specified set of theta values. It can also display the conditional standard error of estimation (CSEE) at the test level.

Usage

# S3 method for class 'info'
plot(
  x,
  item.loc = NULL,
  overlap = FALSE,
  csee = FALSE,
  xlab.text,
  ylab.text,
  main.text,
  lab.size = 15,
  main.size = 15,
  axis.size = 15,
  line.color,
  line.size = 1,
  layout.col = 4,
  strip.size = 12,
  ...
)

Arguments

x: An object of class info obtained from info().
item.loc: A numeric vector indicating which item information functions to plot, specified by item position (e.g., 1 for the first item). If NULL (default), the test information function for the entire test form is plotted.
overlap: Logical. If TRUE, multiple item information functions are plotted in a single panel. If FALSE (default), each item information function is displayed in a separate panel.
csee: Logical. If TRUE, plots the conditional standard error of estimation (CSEE) at the test level. Note that the CSEE plot is only available at the test level, not for individual items. If FALSE (default), item or test information functions are plotted.
xlab.text, ylab.text: Character strings specifying the labels for the x and y axes, respectively. If missing, default labels are used.
main.text: Character string specifying the overall title of the plot. If missing, a default title is used.
lab.size: Numeric value specifying the font size of axis titles. Default is 15.
main.size: Numeric value specifying the font size of the plot title. Default is 15.
axis.size: Numeric value specifying the font size of axis tick labels. Default is 15.
line.color: A character string specifying the color of the plot lines. Default is "#F8766D". Not used when curves are overlaid (overlap = TRUE with item.loc specified), where colors are assigned automatically. See http://www.cookbook-r.com/Graphs/Colors_(ggplot2)/ for available color names.
line.size: Numeric value specifying the thickness of plot lines. Default is 1.
layout.col: Integer. Number of columns to use when faceting multiple item information functions. Used only when overlap = FALSE. Default is 4.
strip.size: Numeric value specifying the font size of the panel (facet) labels. Used only when overlap = FALSE. Default is 12.
...: Additional arguments passed to ggplot2::geom_line() from the ggplot2 package.

Value

A ggplot2 object of the item or test information functions, or of the test-level CSEE when csee = TRUE.

Details

All of the plots are drawn using the ggplot2 package. The object of class info can be obtained from the function info().

Author

Hwanggyu Lim hglim83@gmail.com

Examples


# \donttest{
## Example using a "-prm.txt" file exported from flexMIRT

# Import the "-prm.txt" output file from flexMIRT
flex_prm <- system.file("extdata", "flexmirt_sample-prm.txt", package = "irtQ")

# Read the item parameters and convert them to item metadata
test_flex <- bring.flexmirt(file = flex_prm, "par")$Group1$full_df

# Define a sequence of theta values
theta <- seq(-4, 4, 0.1)

# Compute item and test information values for the given theta values
x <- info(x = test_flex, theta = theta, D = 1, tif = TRUE)

# Plot the test information function
plot(x)


# Plot the item information function for the second item
plot(x, item.loc = 2)


# Plot multiple item information functions, each in a separate panel
plot(x, item.loc = 1:8, overlap = FALSE)


# Plot multiple item information functions in a single panel
plot(x, item.loc = 1:8, overlap = TRUE)


# Plot the conditional standard error of estimation (CSEE) at the test level
plot(x, csee = TRUE)


# }