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]> 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