I looked at the two matrices. The box on the lower right corner of the
eigenvalues matrix is almost black. So, far in my testing, all of the
eigenvalues for the design matrix are much bigger than zero (suggesting no
linear dependency in the design matrix). I am not sure why I am getting the
rank deficiency error. Also, I get the same rank deficiency errors as soon as
I add any interaction terms to the matrix. Here's the matrix:
ev1 ev2 ev3 ev4
1 -15 -33 -5
1 8 0 -9
1 12 26 32
1 14 25 17
1 14 25 -4
1 11 24 49
1 -17 1 -3
1 -19 0 -13
1 -17 -30 -16
1 14 2 -6
1 12 -23 -6
1 -16 -8 -15
1 15 1 17
1 14 8 -9
1 -17 -14 -13
1 -13 -18 7
1 -18 -21 -11
1 11 20 -10
1 16 13 7
Thanks for you help.
Appu
>===== Original Message From "FSL - FMRIB's Software Library"
<[log in to unmask]> =====
>Hi - what you are doing sounds right. Is it possible that by some chance
>the 3 predictor EVs _do_ actually add up to a constant? Have a look at the
>"Covariance" - is the bottom-right square black?
>
>Cheers, Steve.
>
>
>On Wed, 21 Apr 2004, Appu Mohanty wrote:
>
>> Hi,
>>
>> I am trying to do a regression analyses using the functional data as DV
>> and three questionnaire measures as predictors. Since, the EV's are not
>> demeaned at the higher level of analyses, I mean centered (EV value - mean
>> of EV) the EV's beforehand. I then included an EV consisting of 1's
>> (modeling group mean) and three mean centered EV's corresponding to the
>> three predictors in the model. I get an error message stating "at least
>> one EV is a linear combination of other..." I do not get this error if I
>> include only the three EV's corresponding to the predictors and not the EV
>> modeling the group mean. It appears that adding the EV modeling the group
>> mean adds some linear dependency in the model. I am not sure why this is
>> happening or how to proceed next. I would appreciate any suggestions.
>>
>> thanks
>> Appu
>>
>
> Stephen M. Smith DPhil
> Associate Director, FMRIB and Analysis Research Coordinator
>
> Oxford University Centre for Functional MRI of the Brain
> John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
>
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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