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I have the following study design:

  • 1 between-subjects factor (3 levels; group A, B and C): Three distinct subject groups
  • 1 within-subjects factor (3 levels): Response accuracy in a computer task

I analyzed the data with a mixed design repeated measures ANOVA approach. The analysis produced significant main effects for both factors (between and within), but no significant interaction effect.

When I look at the plotted data for the three groups, it is evident that the within-subjects main effect is driven by the data of group A. In this group, I see a linear increase across the levels of the within-subjects factor. Groups B and C do not show this increase.

This descriptive observation is also evident when I run (exloratory) separate one-way repeated measures ANOVAs for each group: There is a significant effect of the within factor for group A, but not for group B and C.

This is a potentially interesting observation. However, the lack of an interaction effect in the initial ANOVA prevents me from performing (i.e. reporting) the subsequent group-specific analysis.

How can I proceed with this? Would it be okay to run a second ANOVA with only two groups, report the potential interaction effect and perform post-hoc tests? Would I have to adjust the p-value?

Thank you for any help / suggestions!

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