From: Doran, Harold <HDoran_at_air.org>

Date: Wed 24 Jan 2007 - 21:18:07 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 14:54:01 2007

Date: Wed 24 Jan 2007 - 21:18:07 GMT

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

*> From: r-help-bounces@stat.math.ethz.ch
**> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Dave Atkins
**> Sent: Wednesday, January 24, 2007 4:08 PM
**> To: R-help@stat.math.ethz.ch
**> Subject: Re: [R] Cronbach's alpha
**>
**>
**> Harold & Weiwei--
**>
**> Actually, alpha *can* go negative, which means that items are
**> reliably different as opposed to reliably similar. This
**> happens when the sum of the covariances among items is
**> negative. See the ATS site below for a more thorough explanation:
**>
**> http://www.ats.ucla.edu/STAT/SPSS/library/negalpha.htm
**>
**> Hope that helps.
**>
**> cheers, Dave
**> --
**> Dave Atkins, PhD
**> Assistant Professor in Clinical Psychology Fuller Graduate
**> School of Psychology
**> Email: datkins@fuller.edu
**> Phone: 626.584.5554
**>
**>
**> 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 at stat.math.ethz.ch
**> > [mailto:r-help-bounces at 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 at 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.
**> >
**> --
**> Dave Atkins, PhD
**> Assistant Professor in Clinical Psychology
**> Fuller Graduate School of Psychology
**> Email: datkins@fuller.edu
**> Phone: 626.584.5554
**>
**> ______________________________________________
**> 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 14:54:01 2007

Archive maintained by Robert King, hosted by
the discipline of
statistics at the
University of Newcastle,
Australia.

Archive generated by hypermail 2.1.8, at Thu 25 Jan 2007 - 05:30:29 GMT.

*
Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help.
Please read the posting
guide before posting to the list.
*