R/confint.xxirt.R

In sirt: Supplementary Item Response Theory Models

## File Name: confint.xxirt.R
## File Version: 0.13


#--- confidence interval for xxirt object
confint.xxirt <- function( object, parm, level=.95,  ... )
{
    c1 <- coef.xxirt(object)
    v1 <- vcov.xxirt(object)
    if ( ! missing(parm) ){
        c1 <- c1[parm]
        v1 <- v1[ parm, parm]
    }

    q1 <- ( 1 - level ) / 2
    q2 <- 1 - ( 1 - level ) / 2
    quant <- stats::qnorm(q2)
    se <- sqrt( diag(v1) )
    res <- data.frame( a=c1-quant*se, b=c1+quant*se )
    colnames(res)[1] <- paste0( round( 100*q1,1 ), ' %')
    colnames(res)[2] <- paste0( round( 100*q2,1 ), ' %')
    rownames(res) <- names(c1)
    return(res)
}