Plot Item Characteristic Curves (ICC) from exametrika

This function takes exametrika IRT output as input and generates Item Characteristic Curves (ICC) using ggplot2. ICC shows the probability of a correct response as a function of ability (theta).

Usage

plotICC_gg(
  data,
  items = NULL,
  xvariable = c(-4, 4),
  title = TRUE,
  colors = NULL,
  linetype = "solid",
  show_legend = FALSE,
  legend_position = "right"
)

Arguments

data: An object of class c("exametrika", "IRT") from exametrika::IRT().
items: Numeric vector specifying which items to plot. If NULL (default), all items are plotted.
xvariable: A numeric vector of length 2 specifying the range of the x-axis (ability). Default is c(-4, 4).
title: Logical or character. If TRUE (default), display an auto-generated title. If FALSE, no title. If a character string, use it as a custom title (only for single-item plots).
colors: Character vector. Color(s) for the curve. If NULL (default), a colorblind-friendly palette is used.
linetype: Character or numeric specifying the line type. Default is "solid".
show_legend: Logical. If TRUE, display the legend. Default is FALSE.
legend_position: Character. Position of the legend. One of "right" (default), "top", "bottom", "left", "none".

Value

A list of ggplot objects, one for each item. Each plot shows the Item Characteristic Curve for that item.

Details

The function supports 2PL, 3PL, and 4PL IRT models:

2PL: slope (a) and location (b) parameters
3PL: adds lower asymptote (c) parameter
4PL: adds upper asymptote (d) parameter

The ICC is computed using the four-parameter logistic model: $$P(\theta) = c + \frac{d - c}{1 + \exp(-a(\theta - b))}$$

Examples

library(exametrika)
result <- IRT(J15S500, model = 3)
plots <- plotICC_gg(result)
plots[[1]] # Show ICC for the first item

combinePlots_gg(plots, selectPlots = 1:6) # Show first 6 items