From: Doran, Harold <HDoran_at_air.org>

Date: Wed 24 Jan 2007 - 18:33:30 GMT

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu Jan 25 21:18:33 2007

Date: Wed 24 Jan 2007 - 18:33:30 GMT

Weiwei

Something is wrong. Coefficient alpha is bounded between 0 and 1, so negative values are outside the parameter space for a reliability statistic. Recall that reliability is the ratio of "true score" variance to "total score variance". That is

var(t)/ var(t) + var(e)

If all variance is true score variance, then var(e)=0 and the reliability is var(t)/var(t)=1. On the other hand, if all variance is measurement error, then var(t) = 0 and reliability is 0.

Here is a function I wrote to compute alpha along with an example. Maybe try recomputing your statistic using this function and see if you get the same result.

alpha <- function(columns){

k <- ncol(columns) colVars <- apply(columns, 2, var) total <- var(apply(columns, 1, sum)) a <- (total - sum(colVars)) / total * (k/(k-1)) a }

data(LSAT, package='ltm')

> alpha(LSAT)

[1] 0.2949972

Harold

> -----Original Message-----

*> From: r-help-bounces@stat.math.ethz.ch
**> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Weiwei Shi
**> Sent: Wednesday, January 24, 2007 1:17 PM
**> To: R R
**> Subject: [R] Cronbach's alpha
**>
**> Dear Listers:
**>
**> I used cronbach{psy} to evaluate the internal consistency and
**> some set of variables gave me alpha=-1.1003, while other,
**> alpha=-0.2; alpha=0.89; and so on. I am interested in knowing
**> how to interpret 1. negative value 2. negative value less than -1.
**>
**> I also want to re-mention my previous question about how to
**> evaluate the consistency of a set of variables and about the
**> total correlation (my 2 cent to answer the question). Is
**> there any function in R to do that?
**>
**> Thank you very much!
**>
**>
**>
**> --
**> Weiwei Shi, Ph.D
**> Research Scientist
**> GeneGO, Inc.
**>
**> "Did you always know?"
**> "No, I did not. But I believed..."
**> ---Matrix III
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide
**> http://www.R-project.org/posting-guide.html
**> and provide commented, minimal, self-contained, reproducible code.
*

>

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu Jan 25 21:18:33 2007

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