Extract3PLMirt {DFIT}

Extract item discrimination, difficulties, and guessing parameters and estimate covariance estimates for 3PL items from a fitted mirt object for one or two groups

Description

Extract item discrimination, difficulties, and guessing parameters and estimate covariance estimates for 3PL items from a fitted mirt object for one or two groups

Usage

Extract3PLMirt(mod, focal = NULL, reference = NULL)

Arguments

mod

A mirt object containing the fit of unidimensional model.

focal

Character. Required if mod is MultipleGroupClass, focal should coincide with the label for the focal group. If mod is SingleGroupClass, it is ignored.

reference

Character. Required if mod is MultipleGroupClass, reference should coincide with the label for the focal group. If mod is SingleGroupClass, it is ignored.

Value

If mod contains any itemtype == "3PL", a list with the item parameters and the estimate covariances (if available). If mod is SingleGroupClass, the list contains the item parameters as a matrix and the covariances as a list. If mod is MultipleGroupClass, the list contains the item parameters and covariances for the focal and reference groups only.

Examples

library(mirt)
data <- expand.table(LSAT7)
(mod1 <- mirt(data, model = 1, itemtype = "3PL", SE = TRUE))
(DFIT:::Extract3PLMirt(mod1))