irt (version 0.2.9)

plot.Item: Plot Item Characteristic Curve of an Item object

Description

plot.Item Plots the item characteristic curve for dichotomous items and category response functions for polytomous items.

Usage

# S3 method for Item
plot(
  x,
  theta_range = c(-4, 4),
  title = "",
  suppress_plot = FALSE,
  category_names = FALSE,
  legend_title = NULL,
  base_r_graph = FALSE,
  ...
)

Value

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

Arguments

x

An Item-class object.

theta_range

Either (a) a numeric vector of length two where the values are minimum and maximum theta values, or, (b) a numeric vector of length more than two where values represents the theta values that will be plotted.

title

Title of the plot. By default if the item is 1-4PM IRT model then the title will be "Item Characteristic Curve" if the item follows Graded Response Model the title will be "Category Response Functions". Set it to NULL to suppress the title.

suppress_plot

If FALSE the function will print the plot. If TRUE, function will return the plot object. Default value is FALSE. Function cannot suppress plot when base_r_graph = TRUE, but graph still can be saved in a variable.

category_names

If the model used is 'GRM' (Graded Response Model) these names will serve as category names. For example, c("Strongly Disagree", "Disagree", "Agree", "Strongly Agree"). The default is FALSE where the default category scores will be printed. If the value is NULL no legend will be printed but the categories will be printed differently.

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.

...

Additional arguments that will be passed to geom_line

Author

Emre Gonulates

Examples

plot(x = item(b = 0.3, D = 1, model = "1PL"))

itm1 <- item(a = 1.2, b = 0.3, c = .2, model = "3PL")
plot(itm1)
plot(item(a = 1.2, b = 0.3, c = .2, d = .89, D = 1))

# Use base R graphics for the plot
plot(itm1, base_r_graph = TRUE)

# Plot Graded Response Model
itm2 <- item(a = 0.902, b = c(-1.411, 0.385, 1.79), model = "GRM")
plot(itm2)
plot(itm2, category_names = c("Strongly Disagree", "Disagree", "Agree",
                              "Strongly Agree"))

plot(itm2, category_names = c("Strongly Disagree", "Disagree", "Agree",
                              "Strongly Agree"), base_r_graph = TRUE)

# A Graded Response Model item with two categories (i.e. 2PL item):
itm3 <- item(a = 0.8, b = 1, model = "GRM")
plot(itm3, category_names = c("Incorrect", "Correct"),
     legend_title = "Response")

if (FALSE) {
# Change the y-axis label (Only available if 'ggplot2' is installed)
# plot(itm3, suppress_plot = TRUE) + ylab("New Label")
}