polytomous item example for multipleGroup() function

[Greta Kurpicz]

Good afternoon everyone!

I am currently experimenting with differential item functioning! The personality trait questionnaire I use is polytomous (likert scale 1-5). I have found the below example, but in my case, it didn't let me proceed the independent, not-constrained model without constraining at least some variables as anchor variables.

Also in other examples I noticed this. I was wondering why in the below example was no need for any anchor items. Has this to do with the simulation of the data?

Many thanks advance for your ideas!

Greta

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#polytomous item example

set.seed(12345)

a <- matrix(abs(rnorm(15,1,.3)), ncol=1)

d <- matrix(rnorm(15,0,.7),ncol=1)

d <- cbind(d, d-1, d-2)

itemtype <- rep('graded', nrow(a))

N <- 1000

dataset1 <- simdata(a, d, N, itemtype)

dataset2 <- simdata(a, d, N, itemtype, mu = .1, sigma = matrix(1.5))

dat <- rbind(dataset1, dataset2)

group <- c(rep('D1', N), rep('D2', N))

model <- 'F1 = 1-15'

mod_configural <- multipleGroup(dat, model, group = group)

plot(mod_configural

[Phil Chalmers]

The model below isn't really for DIF, it's just a multiple group model with effectively no constraints (equivalent to taking a subset of the data for each group and fitting independent single group models). This is historically a starting point for some DIF procedures but requires equating the models first so that the latent trait scales are properly aligned. Hence, as it stands you can't actually use this model for DIF as the item parameters are not on comparable scales. HTH.

Phil

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