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How can I carry out a simple main effects test using either SAS or SPSS?
You can use either SAS or SPSS to conduct these tests. For example, let's say that you had a fairly straightforward completely between-subjects design, with a dependent variable called Y and two categorical predictors, A and B, each with two levels. Suppose that you wanted to test the effect of A for each level of B.
In SAS release 6.11 or higher, you can use the SLICE option in PROC GLM, like this:
PROC GLM ;
CLASS a b ;
MODEL y = a b a*b ;
LSMEANS a*b / SLICE=b;
If you are using SPSS, you can use the MANOVA command to test the same hypotheses as the SAS program shown above:
MANOVA y BY a(1,2) b(1,2)
/design = B A within b(1) A within b(2).
More complex designs are testable with the recent SPSS GLM and SAS MIXED procedures, including within-subjects simple main effects tests.
If you have further questions, send E-mail to stats@ssc.utexas.edu.