upperpar: Extract Upper Asymptote Parameters of Item Response Models

Description

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

Usage

upperpar(object, …)
  # S3 method for raschmodel
upperpar(object, alias = TRUE, vcov = TRUE, …)
  # S3 method for rsmodel
upperpar(object, alias = TRUE, vcov = TRUE, …)
  # S3 method for pcmodel
upperpar(object, alias = TRUE, vcov = TRUE, …)
  # S3 method for plmodel
upperpar(object, alias = TRUE, logit = FALSE, vcov = TRUE, …)
  # S3 method for gpcmodel
upperpar(object, alias = TRUE, vcov = TRUE, …)

Arguments

object

a fitted model object whose upper asymptote parameters should be extracted.

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 raschmodels, rsmodels, pcmodels and gpcmodels, where all upper asymptote parameters are fixed to 1, this means that an empty numeric vector and an empty variance-covariace matrix is returned if alias is FALSE.

logit

logical. If a plmodel of type "3PLu" or "4PL" model has been fit, the upper asymptote parameters were estimated on the logit scale. If logit = FALSE, these estimates and the variance-covariance (if requested) are retransformed using the logistic function and the delta method.

vcov

logical. If TRUE (the default), the variance-covariance matrix of the upper asymptote parameters is attached as attribute vcov.

…

further arguments which are currently not used.

Value

A named vector with upper asymptote parameters of class upperpar and additional attributes model (the model name), alias (either TRUE or a named numeric vector with the aliased parameters not included in the return value), logit (indicating whether the estimates are on the logit scale or not), and vcov (the estimated and adjusted variance-covariance matrix).

Details

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

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

Examples

Run this code

# NOT RUN {
if(requireNamespace("mirt")) {

o <- options(digits = 3)

## load simulated data
data("Sim3PL", package = "psychotools")

## fit 2PL to data simulated under the 3PLu
twoplmod <- plmodel(Sim3PL$resp2)

## extract the upper asymptote parameters (all fixed at 1)
up1 <- upperpar(twoplmod)

## fit 3PLu to data simulated under the 3PLu
threeplmodu <- plmodel(Sim3PL$resp2, type = "3PLu")

## extract the upper asymptote parameters
up2 <- upperpar(threeplmodu)

## extract the standard errors
sqrt(diag(vcov(up2)))

## extract the upper asymptote parameters on the logit scale
up2_logit <- upperpar(threeplmodu, logit = TRUE)

## along with the delta transformed standard errors
sqrt(diag(vcov(up2_logit)))

options(digits = o$digits)
}
# }

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