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).

### See Also

`personpar`, `itempar`, `threshpar`

### 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]