Cronbach’s Alpha & Maximized 4. SAS proc corr nosimple nocorr nomiss alpha; var q1-q10; run; Cronbach Coefficient Alpha VariablesAlpha Raw0.743825.

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Presentation transcript:

Cronbach’s Alpha & Maximized 4

SAS proc corr nosimple nocorr nomiss alpha; var q1-q10; run; Cronbach Coefficient Alpha VariablesAlpha Raw

Cronbach Coefficient Alpha with Deleted Variable Deleted Variable Raw Variables Correlation with TotalAlpha q q q q q q q q q q

The Idealism Scale 7. Deciding whether or not to perform an act by balancing the positive consequences of the act against the negative consequences of the act is immoral. 10. Moral actions are those which closely match ideals of the most "perfect" action.

Alpha from Item Variances If nmiss (of Q1-Q10) > 0 then delete; TOT=Q1+Q2+Q3+Q4+Q5+Q6+Q7+Q8+Q9 +Q10; proc means var; var q1-q10 tot; OUTPUT OUT=VarOut VAR=VQ1-VQ10 VTot;

title 'Computing alpha from item variances.'; DATA _NULL_; FILE PRINT; SET VAROUT; SUMVAR = SUM(OF VQ1-VQ10); Alpha = (10/9)*(1-SUMVAR/VTOT); put; put Alpha = ; run; Alpha=

Working with Only First 4 Items Create all three possible split halves A1=Q1+Q2; B1=Q3+Q4; A2=Q1+Q3; B2=Q2+Q4; A3=Q1+Q4; B3=Q2+Q3; Obtain alpha the easy way proc corr nosimple nocorr nomiss alpha; var Q1-Q4; Alpha =

Find the 3 Split-Half Corrs proc corr nosimple nomiss; var A1; with B1; proc corr nosimple nomiss; var A2; with B2; proc corr nosimple nomiss; var A3; with B3; r 1 = r 2 = r 3 =

Obtain 4 for Each Split Half Apply Spearman-Brown correction The three values are.6629,.7966, and The mean of these three is.72, Cronbach’s alpha. The highest of these three is maximized 4

Osburn (2000) showed that Cronbach alpha is a conservative estimate of reliability. Maximized 4 is a better estimate. But for 2n items, there are pairs of split halves Ouch. For 10 items, 126 split halves. There are ways to estimate, if you have FORTRAN.