Frederik Nagel

I think there may be an error in your code and therefore the function returns an incorrect value. In the last line, you have a=(N*r)/(1+(N-1)*r); I believe that instead of the 1 in the denominator, you should have vbar, where vbar is the average variance (see equation on wikipedia). I checked the results of your function versus the output of SPSS and the output of Alexandros Leontitsis's posted m-file, and your function does not produce either the correct alpha or standardized alpha (Leontitdis's function produces the correct unstandardized alpha).

X =

     2 4 2 5
     7 1 3 4
     7 2 4 7
     4 7 2 5
     6 2 6 6
     1 6 2 4
     3 4 2 4
     7 7 2 6
     6 1 2 1
     7 4 4 1

SPSS Output: Alpha = -0.1235 , 0.0036 (standardized)
Leontitsis Output: cronbach(X), ans = -0.1235
Your m-file Output: cronbach(X), ans = 0.2088