[Freesurfer] questions about group analysis

Hello,

I have some questions about doing group analysis. I know these are really basic... but everyone loves easy softballs, right?

Question 1. How should I define multiple classes?

The tutorial examples show two classes, male and female. If I have patient and controls, in addition to male and female, should I define it :

class Patient class Control class Male class Female Variable Age Input subject1 Patient Male 24 Input subject2 Control Female 39 :

or do I have to boil it down to one column??

class PatientFemale class PatientMale class ControlFemale class ControlMale Variable Age Input subject1 PatientMale 24 Input subject2 ControlFemale 39 :

or should I separate these into different FSGD files?

or some other way?

Question 2: Best practice for a small study

If I have lots of slopes, but not so many samples (<50), is it advisable to make one big FSFD file or make several small FSGD files with all those slopes?

Question 3: DOSS versus DODS

By staring at Doug Greve's excellent slides http:// surfer.nmr.mgh.harvard.edu/docs/ftp/pub/docs/fsgroupana.pdf I was able to figure out whad doss and dods do the design matrix. Can someone make a simple statement as when to use one instead of the other?

Question 4: Multiple Comparison

Section 4 of the tutorial gets into multiple comparison. I started to run the sample and realized it will probably take my computer 3 years to finish executing ;) Can someone tell me what problem this is solving for me? What is multiple comparison and where do you run into trouble by not doing it?

That's all I can think of for now. Thank you.

Ray

PS: I did notice there is a broken link in the wiki tutorial. On the page https://surfer.nmr.mgh.harvard.edu/fswiki/FsTutorial/ GroupAnalysis there is a link in section 3.0 that points to information about creating contrast vectors. It is broken. I think it should point to: https://surfer.nmr.mgh.harvard.edu/fswiki/ FsTutorial_2fCreateContrastVectors