Re: Question about Cronbach's Alpha

Posted by Beckstead, Jason on Jun 01, 2012; 9:59pm
URL: http://spssx-discussion.165.s1.nabble.com/Question-about-Cronbach-s-Alpha-tp5713438p5713478.html

Cronbach's coefficient is a form of the intraclass correlation coefficient (ICC). It is basically telling you the proportion of variance in your two-way 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 k-1 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: [phone]  fax: [phone]          

  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

Subscale	Number of Items	Coefficient Alpha
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.

 

 

 

Confidentiality Notice: This message may contain confidential information intended for the person(s) named above. If you have received this message in error, you are prohibited from disseminating or copying this message and requested to notify the sender immediately. Failure to do so may result in violations of federal and Maryland law.