Generate GRM Item Parameters

Creates a list of discrimination (a) and threshold (b) parameters suitable for passing to irt_design() with model = "GRM".

Usage

irt_params_grm(
  n_items,
  n_categories,
  a_dist = "lnorm",
  a_mean = 0,
  a_sd = 0.25,
  b_mean = 0,
  b_sd = 1,
  seed = NULL
)

Arguments

n_items

Positive integer. Number of items.

n_categories

Positive integer >= 2. Number of response categories per item. Produces n_categories - 1 threshold columns in b.

a_dist

Character string for the discrimination distribution. Currently only "lnorm" is supported. Default: "lnorm".

a_mean

Numeric. meanlog for the log-normal distribution. Default: 0.

a_sd

Numeric. sdlog for the log-normal distribution. Default: 0.25.

b_mean

Numeric. Mean around which thresholds are centered. Default: 0.

b_sd

Numeric. SD of the base threshold distribution. Default: 1.

seed

Optional integer seed for reproducibility.

Value

A named list with elements:

a

Numeric vector of length n_items.

b

Numeric matrix with n_items rows and n_categories - 1 columns. Thresholds are ordered within each row.

See also

irt_params_2pl() for 2PL parameters, irt_design() to use the generated parameters.

Examples

# GRM parameters: 15 items, 5 response categories
params <- irt_params_grm(n_items = 15, n_categories = 5, seed = 42)