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Generate 1PL Item Parameters

Source: R/irt_params_helpers.R
irt_params_1pl.Rd

Creates a list of difficulty (b) parameters suitable for passing to irt_design() with model = "1PL". The 1PL model is Rasch-family: every item shares the same discrimination (fixed at 1), so only b is generated here — the a = 1 contract is applied downstream in the design's validate_params step.

Usage

irt_params_1pl(
  n_items,
  b_dist = "normal",
  b_mean = 0,
  b_sd = 1,
  b_range = c(-2, 2),
  seed = NULL
)

Arguments

n_items

Positive integer. Number of items.

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 a single element b (numeric vector of length n_items). Note: no a is returned — 1PL fixes discrimination at 1 downstream rather than at generation time.

See also

irt_params_2pl() for the free-discrimination binary alternative, irt_design() to use the generated parameters.

Examples

# Default 1PL parameters for 30 items
params <- irt_params_1pl(n_items = 30, seed = 42)

# Evenly-spaced difficulty across a wider range
params <- irt_params_1pl(n_items = 20, b_dist = "even", b_range = c(-3, 3))