R/pcm.conversion.R

In sirt: Supplementary Item Response Theory Models

## File Name: pcm.conversion.R
## File Version: 0.12


#*** conversion of the partial credit parametrization
pcm.conversion <- function( b )
{
    maxK <- rowSums( 1 - is.na(b) )
    I <- nrow(b)
    # define delta
    delta <- b[ cbind( 1:I, maxK ) ] / maxK
    # define tau
    tau <- matrix( NA, nrow=I, ncol=ncol(b) )
    for (kk in 1:I){
        if ( maxK[kk]==1 ){
            tau[kk,1] <- 0
        }
        if ( maxK[kk] > 1){
            for (vv in 1:maxK[kk] ){
                t1 <- 0
                if (vv > 1){
                    t1 <- sum( tau[kk,seq(1,vv-1)] )
                }
                tau[kk,vv] <- b[kk,vv] - vv*delta[kk] - t1
            }
        }
    }
    res <- list( delta=delta, tau=tau )
    return(res)
}