``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82``` ```*(Q) I have ran a series of t tests on groups with unequal ns and would like to compute Cohen's d. I see that SPSS doesn't have option available. How do I compute the ds from the given SPSS output? Is there a macro available? *(A) Posted by Marta to SPSSX-L list on 2002/06/05; improved on 2004/10/08. * I have updated a code I posted to the list 2 years ago. It * calculates Glass, Hedges & Cohen standardised effect sizes, with their * standard errors. These values are then added to the dataset. * Marta 2004/10/08 mailto:biostatistics@terra.es Cohen's d is simply=(mean1-mean2)/Sp. Where Sp is the pooled standard deviation: Sp=sqrt((n1-1)*var1+(n2-1)*var2)/(n1+n2-2)). - var1=variance of sample 1 (Sd1^2) - var2=variance of sample 2 (Sd2^2) If the sample sizes in your study are small, then it's better to use an unbiased estimator called Hedges' g: hedges g=cohen&*(1-(3/(4*(n1+n2)-9))) If variances are unequal, sometimes another standardized mean difference is used, called Glass' d=(mean1-mean2)/Sd2 (Sd2= standard deviation from control group). You can get all the required information from the statistics table in the output. * Improved code (2004/10/08) *. * Example dataset (replace by your own but keep variable names) *. * Data for treatment group go first place, and control group 2nd. DATA LIST LIST /test(A8) meantr sdtr(2F8.2) ntr(F8.0) meanco sdco(2F8.2) nco(F8.0). BEGIN DATA 1 35 9 17 24 8 18 2 43 10 15 37 7 16 3 40 2.3 18 32 2.54 19 END DATA. * Analysis *. MATRIX. GET test /VAR=test. GET mean1 /VAR=meantr. GET sd1 /VAR=sdtr. GET n1 /VAR=ntr. GET mean2 /VAR=meanco. GET sd2 /VAR=sdco. GET n2 /VAR=nco. PRINT {mean1,sd1,n1,mean2,sd2,n2} /FORMAT='F8.1' /TITLE='Original data' /RNAMES=test /CLABELS='Mean1','SD1','N1','Mean2','SD2','N2'. COMPUTE n=n1+n2. COMPUTE poolvar=((n1-1)&*(sd1&**2)+(n2-1)&*(sd2&**2))/(n-2) . COMPUTE glass=(mean1-mean2)/sd2. COMPUTE se_glass=SQRT((n/(n1&*n2))+((glass&**2)/(2&*(n2-1)))). COMPUTE cohen=(mean1-mean2)/SQRT(poolvar). COMPUTE se_cohen=SQRT((n/(n1&*n2))+((cohen&**2)/(2&*(n-2)))). COMPUTE hedges=cohen&*(1-(3/(4&*(n)-9))). COMPUTE se_hedge=SQRT((n/(n1&*n2))+((hedges&**2)/(2&*(n-3.94)))). PRINT {glass,se_glass,cohen,se_cohen,hedges,se_hedge} /FORMAT='F8.3' /RNAMES=test /CLABELS='Glass','SE(D)','Cohen','SE(d)','Hedges','SE(g)' /TITLE='Standardized effects sizes'. COMPUTE data={glass,se_glass,cohen,se_cohen,hedges,se_hedge}. COMPUTE namevec={'glass','seglass','cohen','secohen','hedges','sehedges'}. SAVE data /OUTFILE='c:\\temp\\data.sav' /NAMES=namevec. END MATRIX. MATCH FILES /FILE=* /FILE='c:\\temp\\data.sav'. EXECUTE. ```
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