R/itemFitMirt.R

In joakimwallmark/PolyOptimalIRT:

# Implements S-X^2
# Each item in U starts with 0
# mincell is minimum allowed cell value before collapse
itemFitMirt <- function(mirtModel, data, items = 1:extract.mirt(mirtModel, "nitems"), approxThetaPdf = F) {
    theta_lim <- c(-6, 6)
    qpoints <- seq(-6, 6, length.out = 101)

    probMatList <- list()
    itemDFs <- vector("numeric", length = ncol(data))
    for (item in items) {
        extr <- extract.item(mirtModel, item)
        probMatList[[item]] <- t(probtrace(extr, qpoints))
        # no of parameters is Df for item
        itemDFs[item] <- sum(mirtModel@ParObjects$pars[[item]]@est)
    }

    density <- if (approxThetaPdf)
        cbind(qpoints, density(fscores(hadsAnxietyGrm), n=101)$y)
    else
        cbind(qpoints, dnorm(qpoints))

    return(itemFit(data, probMatList, density, itemDFs))
}