Here is a nice note on sphericity that you may find helpful.
> Hello everyone !! :)
> I had a few questions about ANOVAs and sphericity. I am conducting a 2 way
> ANOVA with one factor (A) having two levels and the other factor (B)
> having 21 levels.
> Q1) In the 'Within Subjects effects' Table I see there is a main effect of
> factor B. One should then go to 'planned comparisons' to see where the
> effect lies right ? However SPSS throws out a 'multivariate tests' output
> just below the 'planned comparisons' table. What does this table mean ?
> Below the table it says
> "Each F tests the multivariate effect of Factor B. These tests are based
> on the linearly independent pairwise comparisons among the estimated
> marginal means"
> What does this table mean ? I'm 90% sure there is no need to look at this
> table, but REALLY want to know what it means.
> Q2) I have a significant interaction effect (A*B) and I used the following
> syntax to see where the difference lied,
> /EMMEANS=TABLES(A*B) COMPARE(B) ADJ(BONFERRONI)
> /EMMEANS=TABLES(B*A) COMPARE(A) ADJ(BONFERRONI)
> The output that SPSS throws out, I should first look at the 'Multivariate
> tests' table and only if that is significant do I see which direction the
> differences lie in the 'Planned comparisons' table right ?
> Q3) While conducting the Mauchly's Test of Sphericity, I realize one
> requires more than 2 levels of a variable for sphericity to be an issue.
> So for factor A I get a value of 1 for Mauchly's W which means sphericity
> is not an issue (and a . in the 'significance' column). However for my
> other factor, I get a 0.000 and no significance value (i.e - once again
> there is . in the significance column). Does that mean sphericity is an
> issue or is not an issue ? If it was an issue, why didn't it just spit out
> some value <0.05 in the significance column?
> Thanks a lot, take care :)
"When all else fails, RTFM."
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