irt_params_2pl: Generate 2PL Item Parameters

In irtsim: Monte Carlo Simulation-Based Sample-Size Planning for Item Response Theory

Generate 2PL Item Parameters

Description

Creates a list of discrimination (a) and difficulty (b) parameters suitable for passing to irt_design().

Usage

irt_params_2pl(
  n_items,
  a_dist = "lnorm",
  a_mean = 0,
  a_sd = 0.25,
  b_dist = "normal",
  b_mean = 0,
  b_sd = 1,
  b_range = c(-2, 2),
  seed = NULL
)

Arguments

n_items	Positive integer. Number of items.
a_dist	Character string for the discrimination distribution. Currently only "lnorm" (log-normal) is supported. Default: "lnorm".
a_mean	Numeric. Mean of the log-normal distribution for a (i.e., meanlog). Default: 0.
a_sd	Numeric. SD of the log-normal distribution for a (i.e., sdlog). Default: 0.25.
b_dist	Character string for the difficulty distribution. One of "normal" or "even". Default: "normal".
b_mean	Numeric. Mean of the normal distribution for b. Only used when b_dist = "normal". Default: 0.
b_sd	Numeric. SD of the normal distribution for b. Only used when b_dist = "normal". Default: 1.
b_range	Numeric vector of length 2. Range for evenly-spaced b values. Only used when b_dist = "even". Default: c(-2, 2).
seed	Optional integer seed for reproducibility. If NULL (default), the current RNG state is used.

Value

A named list with elements a (numeric vector) and b (numeric vector), each of length n_items.

See Also

irt_params_grm() for GRM parameters, irt_design() to use the generated parameters.

Examples

# Default 2PL parameters for 30 items
params <- irt_params_2pl(n_items = 30, seed = 42)

# Evenly-spaced difficulty
params <- irt_params_2pl(n_items = 20, b_dist = "even", b_range = c(-3, 3))

irtsim documentation built on April 24, 2026, 1:07 a.m.