R/brm.irf.R

In alexanderrobitzsch/sirt: Supplementary Item Response Theory Models

## File Name: brm.irf.R
## File Version: 0.11

#--- item response function (discretized) beta response model
brm.irf <- function( Theta, delta, tau, ncat, thdim=1, eps=1E-10 )
{
    TP <- nrow(Theta)
    K <- ncat
    # compute mid points
    mp <- seq( 1 / (2*(ncat) ), 1, 1/ncat  )
    probs <- matrix( 0, nrow=TP, ncol=K )
    eps <- 1E-10
    # compute beta shape parameters
    m1 <- Theta[,thdim] - delta + tau
    m1 <- exp( m1 / 2 )
    m2 <- - Theta[,thdim] + delta + tau
    m2 <- exp( m2 / 2 )
    for (cc in 1:ncat){
        probs[,cc] <- stats::dbeta( mp[cc], shape1=m1, shape2=m2 )
    }
    probs <- probs + eps
    probs <- probs / rowSums(probs)
    return(probs)
}