Posted by
Rich Ulrich on
Apr 12, 2011; 6:36pm
URL: http://spssx-discussion.165.s1.nabble.com/Questions-about-ANOVAs-and-Sphericity-tp4297706p4298921.html
>
> 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?
I would guess that "sphericity is an issue." The only reason that
occurs to me, for why you don't see a tiny p-value for it, is that
(perhaps) it was non-computable owing to a zero value for some variance
or a 1.0 for some correlation.
Sphericity implies that all of the covariances are similar.
If the 21 levels of B represent time or space, I would expect that
contiguous levels have higher correlations, which is one of the most
common violations.
Further, Mauchley's test is not particularly powerful. Followup tests
can be biassed even when Mauchley's gives a p-value of .3 or .4.
A followup style that is frequently recommended for repeated measures
is a set of paired t-tests, so that each test uses its appropriate error.
If I had your data, I might check to compare those t-tests with the
similar tests from the requested contrasts... I don't expect them to
be the same. Differences in the tests would illustrate the severity
of the problem for these data in particular.
Especially if the levels represent time, space, or any other specific
ordering, I would plot the means in order to examine and explain what
is going on.
--
Rich Ulrich
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