Spearman Brown uses a different model than does coefficient alpha. Coefficient alpha is the average of all Guttman-Flanagan split halves, not Spearman Brown. The Spearman Brown assumes parallel items whereas the Guttman-Flanagan assumes essentially tau equivalent items.
Dr. Paul R. Swank, Professor
Health Promotion and Behavioral Sciences
School of Public Health
University of Texas Health Science Center Houston
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Wu, Yow Wu
Sent: Thursday, October 18, 2012 12:28 PM
To: [hidden email]
Subject: Cronbach Alpha
I used both Spearman Brown Prophecy formula and also used the sum of item variance over total variance formula to calculate the Cronbach Alpha.
For a larger data set 6 items 20 cases, the results turned out the same. Then I tried to demonstrate if two items are highly related, then alpha should be very close to 1.
The following data turned out very different results.
V1 v2
40 60
36 56
80 100
55 76
By using average correlation coefficients the result is 1 but using variance approach the result is totally different.
The data set does not have any meaning but why this happened? My questions are
1. Does SPSS use average correlation coefficient approach to calculate alpha?
2. Why sometimes the results match and others does not.
Thanks.
Bill
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