Cronbach Alpha

Posted by Bill Wu on Oct 18, 2012; 5:27pm
URL: http://spssx-discussion.165.s1.nabble.com/Cronbach-Alpha-tp5715740.html

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