Hi Lena,
> I would like to ask, first, whether people do recommend creating my
> own template (and whether I have a sufficient sample size to do so).
> Second, what would be the most appropriate choice of subjects for
> creating a template given my sample size and unbalanced design.
if you want to compare the whole groups with each other (instead of
perhaps homogenizing them in order to have more narrow age brackets), I
think it would be best to create your own templates by using all
subjects, and your numbers are not too bad.
Here is what I would do: do an affine transformation for all subjects,
using the template bounding box. Segment them and clean the segmented
images as described earlier. Use these as your own a-priori-maps for
segmentation (if you want to do a classical VBM study, which you
probably don't) or segmentation and normalization (for the optimized
protocol). Instead of this "classical" way to obtain apriori-maps, you
can also use an optimized approach for creating your tissue templates;
this has pros and cons.
As to the further analyses, you should (next to gender and IQ and what
not) use both age and global gray matter volume as covariates. Note that
there may be different interactions in the groups (age could affect
schizophrenics differently than controls, just as you probably
hypothesize for gender), so perhaps an interaction term would also be
appropriate.
Best to London,
Marko
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Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
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