MANOVA and Box's test

#1
Hi everyone,

I really hope you can help me out with this question. I have a 2x2x2 factorial design with 3 independent variables and 2 dependent variables. Each independent variable has 2 levels. Each dependent variable is a 15-item questionnaire. I'm running a MANOVA with the 3 IV's as factors and the 30 questionnaire items as DV's:

IV1: Hispanic, Anglo
IV2: Hispanic, Anglo
IV3: Activiy1, Activity2
DV1: 6 items with 5-point Likert scale
DV2: 26 items with 5-point Likert scale

I'm using SPSS 14 to run the MANOVA. When I run the procedure, I get the descriptive stats, Levene's test, multivariate and univariate tests, etc., but for Box's test I get this message:

"Box's Test of Equality of Covariance Matrices is not computed because there are fewer than two nonsingular cell covariance matrices."

I couldn't find any information on this in the SPSS help section, or on other forums.
- Any reasons why Box's test would not be generated with this design?
- What is meant by "two nonsingular cell covariance matrices"?
- And more importantly, is there any workaround to get Box's test to work?

Any pointers/ideas/suggestions to solve this issue will be appreciated. Finding a solution would make a great Christmas present :)

Thanks,

Roberto
 
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#3
It's true that not everyone agrees on the importance of Box's test. However, if you are working with a group of people who think the test should be computed and its results are essential to determine the validity of your data, then you don't have much of a choice.

Regarding that (the meaning of the "Box's Test of Equality of Covariance Matrices is not computed" message), the SPSS support website does not address it, i.e., it does not explain exactly what the message means and/or how to work around the issue.

I double checked my data (e.g., that variables were set to "numeric", that fields are filled in with numbers and not alphanumeric characters, etc.), and everything looks OK. The message does not indicate invalid data, though, it seems to say that the design (IV's, DV's, levels) does not allow for the test to be run. Any ideas or pointers on that will be appreciated.

Roberto