Hi everybody,
I'm about to perform several large VBM randomise (from fsl-4.1.2) runs
and would like to clarify a few questions before I dive headlong in do
this.
I've got m+n subjects: m controls and n drug users. I'd like to look
at what effect measures such as duration of drug use and education may
have on GM volume. The drug users can be carved into two subgroups:
short and long term abstinent users.
I had intended to examine separately for each group the correlation of
the variables of interest and GM volume.
I've set up my regressors as follows
Short Long Ctrl Age Short Age Long Age Ctrl DoU Short DoU Long Edu
Short Edu Long Edu Ctrl Abst Short Abst Long
1 0 0 40 0 0 12 0 1 0 0 12 0
0 1 0 0 43 0 0 0.29 0 2 0 0 48
0 1 0 0 32 0 0 4 0 1 0 0 56
1 0 0 43 0 0 17 0 3 0 0 16 0
0 1 0 0 44 0 0 2 0 3 0 0 44
0 0 1 0 0 24 0 0 0 0 2 0 0
0 0 1 0 0 29.9 0 0 0 0 3 0 0
0 0 1 0 0 44.4 0 0 0 0 2 0 0
0 0 1 0 0 37 0 0 0 0 4 0 0
The problem I have is that some of the variables of interest make no
sense for the control users (they are effectively N/As), namely
duration of use and length of abstinence. Hence there are two columns
missing above, 'DoU ctrl' and 'Abst ctrl'. Should I include these
columns in the matrix and if so what what values should I put in them?
Should the values I put in these N/A columns for the control users be
the mean value of all the users since the effect of a covariate is
dictated by the it's distance from the mean?
If I understand this correctly, the first 3 columns, encoding group
membership model the mean. Can I therefore avoid demeaning the other
covariates and not put the -D option on the randomise command line?
Also, I have education encoded as an ordinal, namely
Did not finish high school 1
High school grad 2
Some college 3
College grad 4
Graduate school 5
will the constitute a problem, especially if I have to demean the
variables?
Thanks for your time,
Bye,
--
Dr Colm G. Connolly
School of Psychology and Institute of Neuroscience
The Lloyd Building
University of Dublin
Trinity College, Dublin 2, Éire
Fax: +353-1-671-3183
Please note that electronic mail to, from or within the Trinity
College Dublin, may be the
subject of a request under the Freedom of Information Act.
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