<- file stat 97alpha.html -> In this file: Comparing several alphas Negative alphas?

Comparing alphas

=======================Greg Hancock, 16 May 1997==========ssc Message-ID: <Pine.SOL.3.95q.970516073240.4977A-100000@rac6.wam.umd.edu> From: Greg Hancock <ghancock@WAM.UMD.EDU> Subject: Feldt's test On Thu, 15 May 1997 Jason Thompson wrote: > Does anyone know a reference for Feldt's test for equality of > reliability coefficients? I'm trying to find significant differences > between interrater agreement. A general form of Feldt's test was presented in the following: Hakstian, A. R., & Whalen, T. E. (1976). A k-sample significance test for independent alpha coefficients. Psychometrika, 41(2), 219-231. Note specifically that these are for independent coefficients. If you have dependent coefficients, a different test is needed. Such a test exists and was published in Psychometrika in the 70's or 80's (I believe also by Feldt or Hakstian), but the specific reference escapes me right now. Good luck. =======================Paul Barrett, 16 May 1997==========ssc Message-ID: <E0wSMNS-0001NO-00@pcmail.liv.ac.uk> << faq snip duplication >> Feldt, L.S., Woodruff, D.J., and Salih, F.A. (1987) Statistical Inference for Coefficient Alpha. Applied Psychological Measurement, 11, 1, 93-103 I don't have the paper with me right now - but I think it discusses the comparability of dependent alphas (amongst other issues such as confidence intervals et.)

Negative alpha reported?

=======================Rich Ulrich, 09 Apr 1997==========spss Subject: Re: Cronbach's alpha Message-ID: <5ih0kn$48i@usenet.srv.cis.pitt.edu> If you score one variable the wrong way, each of its correlations are negative. But what is harder for a program to notice is what happens when you score half your correlations in the wrong direction. Then, assuming all the correlations were about equal, your "average correlation", off the diagonal of your correlation matrix, actually becomes negative - if you have two sets of your variables, nxn, where each set is consistent but scored opposite, then there will be n*n NEGATIVE correlations, and (n*(n-1)) positive correlations: more negative than positive. It works out with more negatives, also, if the two sets differ in size only by one variable. I think that I do agree that some warnings should be offered - what Dick Campbell suggests is really quite simple, and would give some protection. It is hard to recommend any step that goes beyond warnings, since there IS some justification, at times, to stick in a variable that has negative correlations (conceptually, some "correction factor"). ===============> concerning note: Dick Campbell (DCAMP@UIC.EDU) wrote: ": .... Thus a negative alpha says not that the software is : incorrect but rather that you have a serious problem, probably resulting : from a directional coding error in one or more of the items. I apologize for : the lack of clarity. Since users frequently calculate alpha without asking for : the item correlation matrix, it might be useful to put in a flag warning the : user if there are negative covariances present. " * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

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