For question #3; #4.

#3
Any good stastistical textbook will be useful. The two that come to mind are:
(a) Meyers and Wells. Research Design and Statistical A nalysis
(b) Keppel and Wickens. Design and Analysis: A Researcher's Handbook (4th Edition)

#4

SPM uses the General Linear Model for all of its second level models. The DODS and DOSS are terms used in the freesurfer package and not in other packages; however, the designs can easily be generated in SPM.

DODS -- Design matrix with different slopes per group
Select two-sample t-test (or flexible factorial)
Specify your two groups of scans
Specify the covariate values, names, and select interaction with factor 1

You can ask if the slopes are different

DOSS -- Design matrix with same slopes across all groups
Select two-sample t-test (or flexible factorial)
Specify your two groups of scans
Specify the covariate values, names, and select interaction none.

The slopes are modelled as a single regressor, so you can't ask about the difference in slopes between groups.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
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On Mon, Aug 22, 2011 at 8:46 AM, Tetiana Dadakova <tetiana.d@gmail.com> wrote:
Hello Douglas,

I did as you suggested and I got the results, but I still have
difficulty understanding how exactly the choice of Design Matrix Type
influences the analysis, so I have several questions:

1. When I look at thickness difference between two groups and choose
age as nuisance parameter, doesn't it mean that calculation take into
account the difference in slopes already (as I correct for age)? So
therefore, when I also choose DODS or DOSS I do kind of double
correction (matrix type + nuisance), don't I?
2. When I choose e.g. DODS and do the analysis, can I see the slopes
(or the values of slopes) for two groups somehow? In which file this
output is?
3. Can you recommend me anything to read regarding mathematics of how
the analysis is done with either matrix type?
4. Just in case you know: how is this step implemented in SPM? It also
uses glm, but it doesn't ask for the DODS vs DOSS choice.

Thank you, I appreciate you time and help,
Tanja.



On Wed, Aug 17, 2011 at 6:19 PM, Tetiana Dadakova <tetiana.d@gmail.com> wrote:
> Thank you, Douglas.
>
> On Wed, Aug 17, 2011 at 5:48 PM, Douglas N Greve
> <greve@nmr.mgh.harvard.edu> wrote:
>>
>> Hi Tanja, you should use DODS and test for a difference between the age
>> slopes of the groups (ie, an interaction between group and age). If there
>> are no significant interactions, you should then use DOSS to test for the
>> difference between thicknesses.
>> doug
>>
>> Tetiana Dadakova wrote:
>>>
>>> Dear all,
>>>
>>> I have two groups of subjects, and I want to see a difference in
>>> thickness between them controlling for age.
>>> I couldn't find any information in the literature on how the subject's
>>> brains in my groups develop with age. Therefore I can't tell whether
>>> thickness as a function of age for both groups has the same slope or
>>> different.
>>>
>>> My question is if in this case I should use DODS?
>>>
>>> Thank you,
>>> Tanja.
>>> _______________________________________________
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>>>
>>>
>>>
>>
>> --
>> Douglas N. Greve, Ph.D.
>> MGH-NMR Center
>> greve@nmr.mgh.harvard.edu
>> Phone Number: 617-724-2358 Fax: 617-726-7422
>>
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