R/probs2logits.R

In alexanderrobitzsch/immer: Item Response Models for Multiple Ratings

## File Name: probs2logits.R
## File Version: 0.06
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# converts probabilities to logits
probs2logits <- function(probs){
    include_zero <- TRUE
    #*** initial values
    NP <- length(probs)
    q1 <- stats::qlogis(probs)
    x0 <- q1-q1[1]
    x0 <- x0[-1]
    #*** define optimization function
    fr <- function(x) {
        NX <- length(x)
        NP <- NX+1
        y <- rep(0,NP)
        y[ seq(2,NX+1) ] <- x
        p1 <- exp(y) / sum( exp(y) )
        p1 <- sum( (probs - p1)^2 )
        return(p1)
    }
    #*** optimization
    a1 <- stats::optim( x0, fr)
    y0 <- a1$par
    if (include_zero){
        y0 <- c(0,y0)
    }
    return(y0)
}
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logits2probs <- function(y){
    x <- exp(y)
    x <- x / sum(x)
    return(x)
}
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