Generate 2PL Item Parameters

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))