Hello John, Darren,

I think your remark about "people who know a bit more" scared pretty 
much everyone off ;) so I start this with the disclaimer that I voice an 
opinion despite, not because of, this remark.

I would agree, though, that using only one of the groups is a problem in 
general. I think everyone should be treated as equally as possible 
before entering an analysis, to the point that "equally unfair" may be 
preferrable. I see Darren's point that you want to see differences "with 
regard to only that group", but I would still be worried that you end up 
comparing apples and oranges (i.e., data processed this and that way, 
and that resulting differences arise from the differences in 
processing). I would thus be hesitant to do this.

The effect of customized priors are difficult to predict a priori, 
although I must admit that I do not know many exhaustive simulations on 
this. Simon Keller concluded several years ago (PMID 15528086) that 
custom templates do not have huge effect on the resulting SPM's, but one 
of my contributions to the paper was to mention that this may not be the 
case for unusual populations, such as children (my case) or elderly 
subjects (your case). Also, the newer segmentation approaches have been 
substantially improved since then, so it is hard to use that data as a 
guidance today. Along those lines: one of the issues you naturally are 
concerned about in the setting of an unusual population is the effect of 
your priors on tissue segmentation. An elegant way out of this may be to 
use the vbm8 approach that will not use priors for segmentation at all, 
but will only use them to achieve spatial normalization. This way, you 
can at least minimize the impact of your priors on the processing, 
whichever way you decide to go.

Finally, having written the paper (PMID 18424084), I naturally think it 
is worth a try to feed all the data into the TOM toolbox and then 
extract those features that best statistically model age and gender 
across your whole population. Having "children" in the title was 
probably the worst decision when writing the manuscript, as the approach 
is applicable in all cases when you have a population that deviates from 
the "standard population" used to construct your prior. With the numbers 
you mentioned, the result should be pretty stable.

So much for my contribution to the confusion :)
Cheers,
Marko


John Ashburner wrote:
> Hi Darren,
> I could be wrong, but isn't it generally better to treat all scans
> equally (ie not use any of the information about the columns of the
> design matrix in which you're interested) if the intention is to make
> statistical inferences about it?  If a template is generated from some
> subjects (selected based on a categorical variable of interest), but not
> others, this would imply that the data are not all treated the same.
>
> I would like to hear what do others who know a bit more about stats
> think about this, particularly in regards to approaches such as lesion
> masking for spatial normalisation.
>
> All the best,
> -John
>
>
>
>
> On 17 October 2013 04:59, Darren G <[log in to unmask]
> <mailto:[log in to unmask]>> wrote:
>
>     Hi
>
>     I have some questions about creating a custom template for a VBM
>     study. Reading through the list, I see that the typical advice is to
>     use all the subjects for creating the template, as opposed to just
>     using the controls, for example. I think this makes sense if
>     subjects are largely from similar groups with equal numbers of
>     subjects represented, but I am wondering what to do if I have about
>     90 healthy controls and about 150 patients. Since the patients do
>     have some atrophy and both groups span a broad age range I think a
>     custom template would help the analysis.
>
>     However, I fear that using all the subjects will bias the template
>     towards the patients. I could take a subset of the patients, but
>     because of the broad age range the choice of which patients to pick
>     is even more difficult.
>
>     Why not use just the controls to create the template? I understand
>     this will bias the template towards the controls, but since I am
>     trying to detect differences between patients and controls this
>     seems appropriate.
>
>     What are the arguments for not just using the controls and is there
>     any literature on this that has studied different ways of making a
>     customized template, i.e., controls only vs. controls + patients?
>
>     Any other suggestions for creating a template with this imbalanced
>     group?
>
>     Thanks
>     Darren
>
>

-- 
____________________________________________________
PD Dr. med. Marko Wilke
  Facharzt für Kinder- und Jugendmedizin
  Leiter, Experimentelle Pädiatrische Neurobildgebung
  Universitäts-Kinderklinik
  Abt. III (Neuropädiatrie)


Marko Wilke, MD, PhD
  Pediatrician
  Head, Experimental Pediatric Neuroimaging
  University Children's Hospital
  Dept. III (Pediatric Neurology)


Hoppe-Seyler-Str. 1
  D - 72076 Tübingen, Germany
  Tel. +49 7071 29-83416
  Fax  +49 7071 29-5473
  [log in to unmask]

  http://www.medizin.uni-tuebingen.de/kinder/epn/
____________________________________________________