psychotools: discrpar – R documentation

Become an expert in R — Interactive courses, Cheat Sheets, certificates and more!

discrpar

Extract Discrimination Parameters of Item Response Models

Description

A class and generic function for representing and extracting the discrimination parameters of a given item response model.

Usage

discrpar(object, ...)
  ## S3 method for class 'raschmodel'
discrpar(object, ref = NULL, alias = TRUE, vcov = TRUE, ...)
  ## S3 method for class 'rsmodel'
discrpar(object, ref = NULL, alias = TRUE, vcov = TRUE, ...)
  ## S3 method for class 'pcmodel'
discrpar(object, ref = NULL, alias = TRUE, vcov = TRUE, ...)
  ## S3 method for class 'plmodel'
discrpar(object, ref = NULL, alias = TRUE, vcov = TRUE, ...)
  ## S3 method for class 'gpcmodel'
discrpar(object, ref = NULL, alias = TRUE, vcov = TRUE, ...)

Arguments

`object`	a fitted model object whose discrimination parameters should be extracted.
`ref`	a restriction to be used. Not used for models estimated via CML as the discrimination parameters are fixed to 1 in `raschmodel`s, `rsmodel`s and `pcmodel`s. For models estimated via MML (`plmodel`s and `gpcmodel`s), the parameters are by default identified via the distributional parameters of the person parameters (mean and variance of a normal distribution). Nevertheless, a restriction on the ratio scale can be applied.
`alias`	logical. If `TRUE` (the default), the aliased parameters are included in the return vector (and in the variance-covariance matrix if `vcov` = TRUE). If `FALSE`, these parameters are removed. For `raschmodel`s, `rsmodel`s and `pcmodel`s where all discrimination parameters are fixed to 1, this means that an empty numeric vector and an empty variance-covariance matrix is returned if `alias` is `FALSE`.
`vcov`	logical. If `TRUE` (the default), the variance-covariance matrix of the discrimination parameters is attached as attribute `vcov`.
`...`	further arguments which are currently not used.

Details

discrpar is both, a class to represent discrimination parameters of item response models as well as a generic function. The generic function can be used to extract the discrimination parameters of a given item response model.

For objects of class discrpar, several methods to standard generic functions exist: print, coef, vcov. coef and vcov can be used to extract the discrimination parameters and their variance-covariance matrix without additional attributes.

Value

A named vector with discrimination parameters of class discrpar and additional attributes model (the model name), ref (the items or parameters used as restriction/for normalization), alias (either TRUE or a named numeric vector with the aliased parameters not included in the return value), and vcov (the estimated and adjusted variance-covariance matrix).

Examples

o <- options(digits = 4)

## load verbal aggression data
data("VerbalAggression", package = "psychotools")

## fit Rasch model to verbal aggression data
rmod <- raschmodel(VerbalAggression$resp2)

## extract the discrimination parameters
dp1 <- discrpar(rmod)

## extract the standard errors
sqrt(diag(vcov(dp1)))

if(requireNamespace("mirt")) {
## fit 2PL to verbal aggression data
twoplmod <- plmodel(VerbalAggression$resp2)

## extract the discrimination parameters
dp2 <- discrpar(twoplmod)

## this time with the first discrimination parameter being the reference
discrpar(twoplmod, ref = 1)

## extract the standard errors
sqrt(diag(vcov(dp2)))
}

options(digits = o$digits)

psychotools

Psychometric Modeling Infrastructure

v0.6-0

GPL-2 | GPL-3

Authors

Achim Zeileis [aut, cre] (<https://orcid.org/0000-0003-0918-3766>), Carolin Strobl [aut], Florian Wickelmaier [aut], Basil Komboz [aut], Julia Kopf [aut], Lennart Schneider [aut] (<https://orcid.org/0000-0003-4152-5308>), Rudolf Debelak [aut]

Initial release

2020-11-16