Tetiana Dadakova 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:
- 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?
If there is a difference between the slopes, then it is very hard to interpret a difference between the thicknesses (some statisticians say that you should not even try). The DOSS will correct for age assuming that the age slopes are the same.
- 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?
If you have two groups and DODS, then the slopes will be the 3rd and 4th frame in the beta.mgz file.
- Can you recommend me anything to read regarding mathematics of how
the analysis is done with either matrix type?
Donald Mclaren gave some good refs.
- 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.
As Donald points out, DODS and DOSS are FS-specific names, but you can implement them in SPM. Sorry, I don't have enough SPM experience to tell you how. doug
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. _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
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