> I have 2 groups of 15 subjects for a VBM study and made the preprocessing.
> Now I'd like to do the stats and I'm a little bit lost to covariate with age and sex.
> How to put the vectors for sex (-1; 1?) and age (difference with the mean?)
> Is the order of the value the order of the subjects successively in the 2 groups (the 16th value is the one for the 1st subject of the 2d group for example)?
It sounds as if you want to compare tissue volume in two groups while
controlling for age and sex differences. To start with, let's say you
just want to control for age differences. In this case you would
specify a 2-sample t-test and add a covariate for age; this will give
you 3 columns in your design matrix (group 1, group 2, age).
In this case, as with any covariate, you need one value per image, and
these values are in the same order as the images you select. So in
this case, you've selected 30 images (15 group 1, 15 group 2). You
are exactly right that the 16th value in the age covariate should be
the age for the 16th image = first subject in group 2. You will want
to mean center the covariate (which is the default in SPM8).
To test the difference between group 1 and group 2, you could then do
a t-test of:
[1 -1 0] (group 1 > group 2)
[-1 1 0] (group 2 > group 1)
You could look at the effect of age with [0 0 1] or [0 0 -1].
In the case where you want to control for sex differences, one common
approach is to model males and females separately within each group,
which would require a slightly different model. However, since you
only have 15 subjects per group, I don't know that this will be
A general comment with regard to covariates is that if your groups are
fairly evenly matched, then including an additional covariate will
reduce the error (which is good) but probably not change the overall
result. If your groups are not particularly well balanced, then your
group difference is confounded with these other factors, and including
them as covariates cannot undo this.
Hope this helps!
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104