SIMPLE MAIN EFFECTS
Click here
to find out how to use EMMEANS (Estimated Marginal Means) in GLM to test simple main effects. You can additionally also add ADJ(BONFERRONI) or ADJ(SIDAK) to the /EMMEANS subcommand line to correct for multiple t-tests and use /EMMEANS for designs involving only between subjects factors (see some examples here and in Field, (2005), in Chapter 11 on Repeated Measures designs.)
Simple effects involving between subject factors can also be specified in SPSS using the /LMATRIX subcommand and the /MMATRIX subcommand for within subjects factors or combinations of the two. See also.
MANOVA can also be used to compare pairs of within subjects factor levels in each group separately using the overall Mean Square Error which was used for testing the original interaction which was computed using all the groups. For example to decompose the Time x A interaction separately into the differences in times in groups 1 and 2 we can use the below. Further details are available here.
MANOVA V1 V2 BY GROUP(1,2) /WSFACTORS=TIME(2) /DESIGN=MWITHIN GROUP(1), MWITHIN GROUP(2).
See David Nichols's illustrations using the above SPSS syntax for explaining all types of interactions here. There are also illustrations of simple effects in the ANOVA Grad talk and some SPSS syntax here for two-way interactions.
Reference
Field, A (2005). Discovering statistics using SPSS. Second Edition. Sage:London.
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These pages are maintained by Ian Nimmo-Smith and Peter Watson
[Last updated on 14 April, 2004]