Hi Rodrigo,
The -D option in randomise has to do with whether the input data is to
be demeaned -- it has nothing to do with whether the regressors in the
design matrix are demeaned.
Quoting the web help page for randomise:
The -D option tells randomise to demean the data before continuing -
this is necessary if you are not modelling the mean in the design matrix
In your case, your design matrices are not modeling a mean, so the -D
option must be included.
cheers,
-MH
On Mon, 2011-11-28 at 19:32 +0000, Rodrigo Perea wrote:
> Could someone explain me this. So I have a column that I want to use as a regressor in my TBSS analysis. I created a matrix without demeaning this value and I got a significant value at p<0.05 of what I expected. Then I demeaned the regressor and also I used the -D option and after 5000 permutations no significance was found in any direction. Why? If someone could help guide me in the right direction, I would greatly appreciate it.
>
>
> Thanks in advance.
> Rodrigo
>
> My non-demeanded design and contrast matrix are (I used this same matrix when using the -D option):
>
> design.mat (sex and age as covariates):
> /Matrix
>
> Sex Age Regressor Variable
> 1.000000e+00 7.500000e+01 1.634750e+03
> 0.000000e+00 7.500000e+01 1.153250e+03
> 0.000000e+00 8.300000e+01 3.151000e+03
> 0.000000e+00 7.300000e+01 1.651000e+03
> 0.000000e+00 8.100000e+01 1.503250e+03
> 1.000000e+00 7.100000e+01 1.325500e+03
> .
> .
> .
>
> design.con:
> /Matrix
> 0.000000e+00 0.000000e+00 1.000000e+00
> 0.000000e+00 0.000000e+00 -1.000000e+00
>
>
>
> My demeaned design matrix and contrast are:
>
>
> design.mat (sex and age as covariates):
> /Matrix
>
> Age Sex Regressor
> 1.000000e+00 7.500000e+01 -6.570900e+02
> 0.000000e+00 7.500000e+01 -1.138590e+03
> 0.000000e+00 8.300000e+01 8.591600e+02
> 0.000000e+00 7.300000e+01 -6.408400e+02
> 0.000000e+00 8.100000e+01 -7.885900e+02
> 1.000000e+00 7.100000e+01 -9.663400e+02
> .
> .
> .
> '
>
> design.con:
> /Matrix
> 0.000000e+00 0.000000e+00 1.000000e+00
> 0.000000e+00 0.000000e+00 -1.000000e+00
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