Given an internal mirt object extracted from an estimated model compute the expected value for an item given the ability parameter(s).
expected.item(x, Theta, min = 0)an extracted internal mirt object containing item information (see extract.item)
a vector (unidimensional) or matrix (multidimensional) of latent trait values
a constant value added to the expected values indicating the lowest theoretical category. Default is 0
Chalmers, R., P. (2012). mirt: A Multidimensional Item Response Theory Package for the R Environment. Journal of Statistical Software, 48(6), 1-29. doi:10.18637/jss.v048.i06
mod <- mirt(Science, 1)
extr.2 <- extract.item(mod, 2)
Theta <- matrix(seq(-6,6, length.out=200))
expected <- expected.item(extr.2, Theta, min(Science[,1])) #min() of first item
head(data.frame(expected, Theta=Theta))
#> expected Theta
#> 1 1.013391 -6.000000
#> 2 1.014407 -5.939698
#> 3 1.015498 -5.879397
#> 4 1.016672 -5.819095
#> 5 1.017933 -5.758794
#> 6 1.019289 -5.698492