Cronbach's coefficient is a form of the intraclass correlation coefficient (ICC). It is basically telling you the proportion of variance in your twoway
table of data (subjects x items) that is due to individual differences among the subjects. Although a form of alpha can be calculated from a correlation matrix, in its original form it is calculated from a covariance matrix.
So, if you have k=10 items and a bunch of subjects who have answered all items, first we calculate a covariance matrix among the 10 items, which has the
item variances in its diagonal. To get alpha, first sum of the diagonal elements (variances) and divide this sum by the sum of all the elements in the matrix. Second, this ratio is subtracted from 1.0. Third, multiple by a weighting factor of k over k1 or
10/9.
hth.
_____________________________________________________________
Jason W. Beckstead, Ph.D.
Associate Professor/Quantitative Methodologist
University of South Florida College of Nursing
12901 Bruce B. Downs Blvd., MDC22, Tampa, FL 33612, USA
Statistical Editor, International Journal of Nursing Studies
phone: +1.813.974.7667 fax: +1.813.974.5418
personal website:
http://personal.health.usf.edu/jbeckste/
International Journal of Nursing Studies
http://www.elsevier.com/ijns
From: SPSSX(r) Discussion [mailto:[hidden email]]
On Behalf Of Kreischer,Resha M
Sent: Wednesday, May 30, 2012 2:02 PM
To: [hidden email]
Subject: Question about Cronbach's Alpha
First off, what is the correct pronunciation for Cronbach?
J
I have utilized SPSS to analyze the reliability of survey subscales by going to Scale/Reliability Analysis/Alpha. Below is the resulting output. Is this enough analysis to determine the reliability of a survey or do I need to do more?
If so, what? What is the best way to describe what the alpha value means in lay person terms?
Parent Survey



Communicating

7

.843

Parenting

5

.822

Student Learning

8

.915

Volunteering

5

.814

School Decision Making

5

.862

Collaborating with Community

5

.818

School Climate

7

.818

·
Coefficient values range between 0 and 1.
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