I was wondering why there is an option for non-sphericity correction for the
paired t-test.
The way I always understood it, the sphericity assumption can only be
violated when more than 2 factor levels are entered into a repeated measures
analysis. in the case of the paired t-test, this is not the case (k=2 levels).
Thx for any comments, and have a good weekend
Claus
P.S.: to clarify things: I use the paired t-test in a rfx analysis, entering
the contrast images of two conditions for 18 subjects as measurement pairs.
the sphericity assumption states that the variance of the difference scores
in a within-subjects design (Var(diff) in a paired t-test) are equal across
all the groups; as there is only one such Variance in a paired t-test, this
assumoption can"t be violated, no? or am I missing sth. here?
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