Re: Stratified anova vs interaction effect

Posted by Bruce Weaver on Dec 05, 2014; 11:19pm
URL: http://spssx-discussion.165.s1.nabble.com/Stratified-anova-vs-interaction-effect-tp5728125p5728126.html

You said, "We hypothesized that T1 would have a stronger effect in Men versus women."  I'm not sure what you mean.  Given the description of the design, I would have expected you to say that the difference between T1 and T2 is expected to be larger in men than in women.  

If you really are talking about the T2-T1 difference, then your stratified analysis shows that the T2-T1 difference is significantly different from 0 for men, but not for women.  That is not the same thing as showing that the two T2-T1 differences (for M and F) are not the same.  If your hypothesis is about the interaction, you must use the F-test for the interaction, not the results of the stratified analysis.

HTH.

parisec wrote

Hi all,

Study design: 2 groups (T1, T2), fixed factor: sex (M, F)

We hypothesized that T1 would have a stronger effect in Men versus women. I plotted means and they  clearly indicated an interaction. i stratified the analysis by sex and ran separate ANOVAs, it was clear that there was an effect in Men (p <0.02) but not in Women. I should have quit there!

But i didn't,  i ran a factorial anova that included group, sex, and the group*sex interaction term. I added the simple effects of group within sex.

  /EMMEANS=TABLES(sex*group) COMPARE (group) ADJ(LSD)

The main effect was just above significance (0.056) and interaction term was not significant, yet the simple effects term showed strong statistical significance, which i know should not be interpreted without a significant interaction.

Now i'm at a loss. The stratified analysis clearly indicates an interaction yet the formal test for interaction didn't pan out.

Is it wrong to just report the results of a stratified analysis?

Thanks for your thoughts.
Carol