R/brxx_ICC_general.R

In brxx: Bayesian Test Reliability Estimation

#'brxx_ICC_general: Bayesian Estimation of Reliability from ICC, General Form
#'
#'This function estimates reliability from intraclass correlation given correlation.
#'
#'@param WS Within subjects variance estimate.
#'@param Resid Residual variance estimate.
#'@param alpha Prior true score variance.
#'@param beta Prior error variance.
#'@param CI Credible interval quantile, as a decimal (ie, for 95 percent, 0.95).
#'@param items Number of test items.
#'
#'@import MCMCpack
#'@import MASS
#'@import TeachingDemos
#'
#'@return Returns estimated median and quantile based credible limits for reliability.
#'
#'@examples
#'WS=20.4
#'Resid=3.6
#'alpha=3.51
#'beta=1.75
#'brxx_ICC_general(WS=WS,Resid=Resid,alpha=alpha,beta=beta,CI=0.95,items=5)
#'
#'@export

brxx_ICC_general=function(WS,Resid,alpha,beta,CI,items){
  CI=ifelse(missing(CI),0.95,CI)
  CI=ifelse(CI>1,CI/100,CI)
  items=ifelse(missing(items),1,items)
  am=ifelse(missing(alpha),1,0)
  bm=ifelse(missing(beta),10,0)
  if(am+bm==0){alpha=alpha
  beta=beta}
  if(am+bm==1){alpha=(0.6*beta)/0.4
  beta=beta}
  if(am+bm==10){alpha=alpha
  beta=0.4*alpha/0.6}
  if(am+bm==11){alpha=3.51
  beta=1.75}
  ll=(1-CI)/2
  ul=1-ll
  Out=round(c(qbeta(c(0.5),alpha+WS*items,beta+Resid*items),
              hpd(qbeta,shape1=alpha+WS*items,shape2=beta+Resid*items,conf=CI)[1],
              hpd(qbeta,shape1=alpha+WS*items,shape2=beta+Resid*items,conf=CI)[2]),4)
  names(Out)=c("Median","LL","UL")
  return(Out)
}