plot.Itempool {irt}

Plot Item Characteristic Curves or Test Characteristic Curve of an Itempool object

Description

plot.Itempool plots the item characteristic curves (item response curves) or test characteristic curve of an Itempool-class object.

Usage

## S3 method for class 'Itempool'
plot(
  x,
  theta_range = c(-4, 4),
  type = "icc",
  tcc_prop_corr = FALSE,
  focus_item = NULL,
  title = "",
  suppress_plot = FALSE,
  legend_title = NULL,
  base_r_graph = FALSE,
  y_lim = NULL,
  ...
)

Arguments

x

An Itempool-class object.

theta_range

Either a numeric vector of length two setting the boundaries of x-axis, e.g. c(-4, 4), or, a numeric vector that is includes the theta values that will be plotted, e.g. seq(-3, 3, by = 0.1).

type

The type of the graph. The default value is "icc". Available options are:

"icc"

Plot item characteristic curve of each item

"tcc"

Plot test characteristic curve

"hist"

Plot histograms of item parameters

"pars"

Plot dot plot of item parameters

tcc_prop_corr

If TRUE, test characteristic curve will be show the proportion correct of the test (i.e. the range of y-axis will be 0-1 instead of 0 to the number of items).

focus_item

A character string of the 'item_id' of the item to be focused. If type = "pars", this item will be shown with a red dot to distinguish it from others.

title

Title of the plot. Set title = NULL to suppress the plot title. The default is "". If type = "tcc" and title = "", title will be 'Test Characteristic Curve'. If type = "icc" and title = "", title will be 'Item Characteristic Curve'. If type = "hist" or type = "pars" and title = "", title will be 'Parameter Values'.

suppress_plot

If FALSE the function will print the plot. If TRUE, function will return the plot object. Default value is FALSE.

legend_title

The title of the plot's legend.

base_r_graph

If TRUE function will plot graphs using base R graphics. If FALSE the function will check whether 'ggplot2' package is installed. If it is installed, it will use 'ggplot2' package for the plot. The default value is FALSE.

y_lim

A numeric vector of length two representing the lower and upper bound of y-axis.

...

Additional arguments that will be passed to geom_line

Value

Depending on the value of suppress_plot function either prints the item characteristic curve or returns the plot object.

Author(s)

Emre Gonulates

Examples

ip <- generate_ip(n = sample(10:15,1))
plot(ip)

# Additional arguments will passed to geom_line
plot(ip, size = .25, alpha = 0.3)

# Set the boundaries of the graph
plot(ip, theta_range = c(-2, 2))
# alternatively provide theta values
plot(ip, theta_range = seq(-6, 6, by = 0.25))


# Test Characteristic Curve
plot(ip, type = "tcc")

# Proportion correct for test characteristic curve
plot(ip, type = "tcc", tcc_prop_corr = TRUE)

# Plot histogram of item parameters
plot(ip, type = "hist")


## Not run: 
# Item parameter summary
ip <- generate_ip(n = 200)
plot(ip, type = "pars")
plot(ip, type = "pars", dotsize = .75)
plot(ip, type = "pars", focus_item = "Item_22")
# Use base R graphics
plot(ip, type = "pars", base_r_graph = TRUE)

# # Remove the legend altogether
# plot(ip, suppress_plot = TRUE) + ggplot2::theme(legend.position="none")
# # Change the labels:
# plot(ip, suppress_plot = TRUE) +
#   ylab("Probability") + xlab("Ability Score")

## End(Not run)