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: [EMAIL PROTECTED] 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: [EMAIL PROTECTED] Phone: 626.584.5554 ______________________________________________ [email protected] 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.
