R/MDISC.R

In xzhaopsy/MIRT: Multidimensional Item Response Theory

#' Compute multidimensional discrimination index
#'
#' Returns a vector containing the MDISC values for each item in the model input object (Reckase, 2009).
#'
#' @aliases MDISC
#' @param x an object of class 'SingleGroupClass'
#'
#' @author Phil Chalmers \email{rphilip.chalmers@@gmail.com}
#' @references
#' Chalmers, R., P. (2012). mirt: A Multidimensional Item Response Theory
#' Package for the R Environment. \emph{Journal of Statistical Software, 48}(6), 1-29.
#' \doi{10.18637/jss.v048.i06}
#'
#' Reckase, M. D. (2009). Multidimensional Item Response Theory. Springer.
#'
#' @seealso \code{\link{extract.group}}
#'
#' @keywords discrimination
#' @export MDISC
#' @examples
#' \dontrun{
#'
#' mod <- mirt(Science, 2)
#' MDISC(mod)
#'
#' }
MDISC <- function(x){
    if(missing(x)) missingMsg('x')
    stopifnot(class(x) == 'SingleGroupClass')
    ret <- numeric(extract.mirt(x, 'nitems'))
    for(i in seq_len(length(ret))){
        item <- extract.item(x, i)
        as <- ExtractLambdas(item)
        ret[i] <- sqrt(as %*% as)
    }
    names(ret) <- extract.mirt(x, 'itemnames')
    ret
}