Hi everyone,
I am looking help to solve the mystery in calculating cronbach's alpha WITHOUT "Nomiss" options in proc corr.
Here is my sample data:
and here is my code:
Proc Corr Data = test Alpha Out = temp1;
Run;
Then I got this result:
I have one missing value on X2.
I didn't use the "Nomiss" option and get a raw alpha of 0.61538.
However, if calculate the alpha manually based on the formula:
Alpha = K/(K - 1) * (1 - (Variance_X1 + Variance_X2) / Variance_(X1+X2)),
Since Variance_X1 = ((1-2)^2 + (2-2)^2 + (3-2)^2) / (3-1) = 1,
Variance_X2 = ((1-1.25)^2 + (1.5-1.25)^2) / (2-1) = 0.125,
Variance_(X1+X2) = ((2-2.83)^2 + (3.5-2.83)^2 + (3-2.83)^2) / (3-1) = 1.125,
and with K = 2,
I get alpha = (2/(2-1)) * (1 - (1+0.125)/1.125) = 0.
Does anyone know how SAS calculate the alpha in this example?
Thank you in advance!
I think you're variance of (X1+X2) formula is incorrect, I get 0.875 but I suppose that depends on how you deal with the missing value, because you'd need that to calculate the Cov(XY).
Var[X + Y] = Var
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