R/probgenerator.R

In changxiulee/BayesianRasch: Bayesian Rasch Modeling

#' probgenerator
#'
#' Generate probability of rating each category given difficulty and ability
#'
#' @param item difficulty of an item
#' @param person ability of a subject
#' @param obj An RSM object or PCM object

probgenerator = function(item, person, obj){
  if(class(obj) == "BPCM"){
    eta.length = length(item) + 1
  }
  else{
    thresh = andrich.threshold(obj)
    eta.length = length(thresh) + 1
  }

  prob = rep(NA, eta.length)
  eta = rep(NA, eta.length)
  psum = rep(NA, eta.length)
  exp.psum = rep(NA, eta.length)
  eta[1] = 0

  for(i in 2:eta.length) {
    if(class(obj) == "BPCM"){eta[i] = person - item[i - 1]}
    else{eta[i] = person - (item + thresh[i - 1])}
  }
  for(i in 1:eta.length){
    psum[i] = sum(eta[1:i])
    exp.psum[i] = exp(psum[i])
  }
  for(i in 1:eta.length){
    prob[i] = exp.psum[i] / sum(exp.psum)
  }
  prob
}