R: Extract Discrimination Parameters of Item Response Models

discrpar {psychotools}

R Documentation

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, ...)

Arguments

`object`	a fitted model object whose discrimination parameters should be extracted.
`ref`	a restriction to be used to ensure parameter identifiability of the item discrimination parameters. Currently not used as the item discrimination parameters are fixed to unity in `raschmodel`'s, `rsmodel`'s and `pcmodel`'s.
`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 item discrimination parameters are fixed to unity, this means that an empty numeric vector and an empty variance-covariace 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

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

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

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

## extract the variance-covariance matrix
head(vcov(dp))

[Package psychotools version 0.4-2 Index]