I have two factors: time and intervention. Time has 4 levels (week 0, 4, 8, and 12), while the intervention consists of intervention group vs control group. The dependent variable is a test score. I have done two-way repeated measures ANOVA and found that there was significant time effect and interaction effect between time and intervention on the test score, but there was no significant main effect between intervention vs control. Now, I want to do a post-hoc analysis to see at which time point the score differences between intervention vs control group is significant. How should I perform the post-hoc test on SPSS? Please correct me if any of the steps that I have mentioned above are wrong. Thanks a lot.
The best approach is to use the emmeans or lsmeans function in your software. This takes into account the model as you have fit it.
For SPSS, see for example: https://www.ibm.com/docs/en/spss-statistics/29.0.0?topic=models-linear-mixed-em-means
I don't use SPSS, so I don't know if it will be able to answer the specific question you are asking, but, likely, there is a way to do a slice among time points.
Sal Mangiafico is right, although he pointed to documentation for the MIXED command, whereas I think you may be using the GLM command. In any case, you need to add a sub-command like this:
/EMMEANS = TABLES(Group*Time) COMPARE(Group)
Replace Group and Time with the names you used if I did not guess them correctly. And if you wish, you can add ADJ(method) where method = LSD (no adjustment), BONFERRONI, or SIDAK.
- https://www.ibm.com/docs/en/spss-statistics/29.0.0?topic=univariate-emmeans-subcommand-glm-command#syn_glm_univariate_emmeans
This is the documentation for EMMEANS under GLM (univariate), but it is the same for GLM REPEATED MEASURES.
Bruce Weaver , in SPSS, is there like a slice option (SAS term) ? Or in R, ~ Group | Time. Where it compares Group within each Time ? I think the results may be different than with Group*Time.
Thank you Bruce Weaver and Sal Mangiafico for your answers, I think I already get the point.
Sal Mangiafico, the EMMEANS sub-command I posted above does generate tests of the simple effects of Group within each level of Time.
/EMMEANS = TABLES(Group*Time) COMPARE(Group)
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