ExtractRaschMirt: Extract item difficulties and item difficulty variance...

In herulor/DFIT: Differential Functioning of Items and Tests

Extract item difficulties and item difficulty variance estimates for Rasch items from a fitted mirt object for one or two groups

Description

Extract item difficulties and item difficulty variance estimates for Rasch items from a fitted mirt object for one or two groups

Usage

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

Arguments

mod	A mirt object containing the fit of unidimensional model.
focal	Character. See description of 'reference'.
reference	Character. If mod is of class MultipleGroupClass and parameters from precisely two groups are to be extracted, 'reference' should coincide with the label for the reference group. If mod is of class MultipleGroupClass and parameers from all groups are to be extracted, 'reference' should be NULL. If mod is SingleGroupClass, it is ignored.

Value

If mod contains any itemtype == "Rasch", 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 = "Rasch", SE = TRUE))
(DFIT:::ExtractRaschMirt(mod1))

# From mirt's multipleGroup
set.seed(12345)
a <- matrix(abs(rnorm(15, 1, .3)), ncol = 1)
d <- matrix(rnorm(15, 0, .7), ncol = 1)
itemtype <- rep('2PL', nrow(a))
N <- 1000
dataset1 <- simdata(a, d, N, itemtype)
dataset2 <- simdata(a, d, N, itemtype,
                    mu = .1, sigma = matrix(1.5))
dataset3 <- simdata(a, d, N, itemtype,
                    mu = -.1, sigma = matrix(1.5))
dataset4 <- simdata(a, d, N, itemtype,
                    mu = 0, sigma = matrix(1.5))

dat <- rbind(dataset1, dataset2, dataset3, dataset4)
group <- rep(paste0('D', 1:4), each = N)

(mod2 <- multipleGroup(dat, 1, itemtype = 'Rasch', group = group, SE = TRUE))
(DFIT:::ExtractRaschMirt(mod2, focal = "D1", reference = "D4"))
(DFIT:::ExtractRaschMirt(mod2))

herulor/DFIT documentation built on June 25, 2024, 8:23 a.m.