cronbachs alpha - score.items(psych) vs reliability(Rcmdr)

Dear Andreas,

I don't know whether there's another comparable function, but reliability()
from the Rcmdr package is very simple; here it is printed out (as you could
have done simply by typing its name -- I added the assignment arrow):

reliability <-
function (S)
{
    reliab <- function(S, R) {
        k <- dim(S)[1]
        ones <- rep(1, k)
        v <- as.vector(ones %*% S %*% ones)
        alpha <- (k/(k - 1)) * (1 - (1/v) * sum(diag(S)))
        rbar <- mean(R[lower.tri(R)])
        std.alpha <- k * rbar/(1 + (k - 1) * rbar)
        c(alpha = alpha, std.alpha = std.alpha)
    }
    result <- list()
    if ((!is.numeric(S)) || !is.matrix(S) || (nrow(S) != ncol(S)) ||
        any(abs(S - t(S)) > max(abs(S)) * 1e-10) || nrow(S) <
        2)
        stop(gettextRcmdr("argument must be a square, symmetric, numeric
covariance matrix"))
    k <- dim(S)[1]
    s <- sqrt(diag(S))
    R <- S/(s %o% s)
    rel <- reliab(S, R)
    result$alpha <- rel[1]
    result$st.alpha <- rel[2]
    if (k < 3) {
        warning(gettextRcmdr("there are fewer than 3 items in the scale"))
        return(invisible(NULL))
    }
    rel <- matrix(0, k, 3)
    for (i in 1:k) {
        rel[i, c(1, 2)] <- reliab(S[-i, -i], R[-i, -i])
        a <- rep(0, k)
        b <- rep(1, k)
        a[i] <- 1
        b[i] <- 0
        cov <- a %*% S %*% b
        var <- b %*% S %*% b
        rel[i, 3] <- cov/(sqrt(var * S[i, i]))
    }
    rownames(rel) <- rownames(S)
    colnames(rel) <- c("Alpha", "Std.Alpha", "r(item, total)")
    result$rel.matrix <- rel
    class(result) <- "reliability"
    result
}

As an alternative to loading the package, you could just put the function
definition in a file -- editing out the calls to gettextRcmdr, which are for
translation of the error messages -- and source() the file when you want to
use it. You'll probably also want the print method (obtained by
Rcmdr:::print.reliability):

print.reliability <-
function (x, digits = 4, ...)
{
    cat(paste("Alpha reliability = ", round(x$alpha, digits),
        "\n"))
    cat(paste("Standardized alpha = ", round(x$st.alpha, digits),
        "\n"))
    cat("\nReliability deleting each item in turn:\n")
    print(round(x$rel.matrix, digits))
    invisible(x)
}

I hope this helps,
 John

------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
web: socserv.mcmaster.ca/jfox

 -----Original Message-----
 From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]

On

 Behalf Of achristoffersen
 Sent: February-03-09 6:47 PM
 > To: r-help at r-project.org
 Subject: [R] cronbachs alpha - score.items(psych) vs reliability(Rcmdr)


 Dear all,

 I like the way the Rcmdr package computes reliability. E.g

 reliability(cov(d[,c("q1", "q2", "q3", "q4", "q5", "q6")],
 use="complete.obs"))

 will not only give me the alpha score, but also for each variable,
 alpha.score if deleted. However - when writing scripts it's very tiresome

to

 load the whole Rcmdr GUI just for this purpose. So I'm looking for an
 another package that delivers the same feature.

 the score.items function in the psych package i find is too complicated

(it

 requires a keys vector) and it doesn't report the "alpha if deleted"

score.

 What have I missed when googling for an alternative?

 Thx in advance

 Andreas
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