GLM Repeated measures Mauchly's test undefined

Posted by praznin on Mar 27, 2013; 2:47am
URL: http://spssx-discussion.165.s1.nabble.com/GLM-Repeated-measures-Mauchly-s-test-undefined-tp5719101.html

I'm doing a two-way mixed ANOVA in SPSS. I have two groups with 9 subjects each (so total = 18), and 24 levels of one measure.

I understand why Mauchly's test of Sphericity has no meaning when there are are only 2 levels of a repeated measures factor, but I notice (using GLM.....repeated measures in SPSS) that Mauchly's test of Sphericity also appears to be undefined (or gives the useless output of Mauchly's W = '.0' , p = '.') when the number of levels of a repeated measure is equal to or greater than the number of cases (subjects). In these instances, even though Mauchly's statistic is not calculated, Greenhouse-Geisser, Huynh-Feldt, and Lower-Bound Epsilon values are calculated.

I would be really happy if someone could provide some insight on why Mauchly's statistic is not calculated in these cases and what should be done to assess sphericity in the absence of Mauchly's statistic.

Thank you