Hi Clara,

the SPC (or any percent change) could be problematic, as the base value (either the volume at the baseline timepoint, or the average volume) will change depending wich interval you look at. You could check if the rate (mm/time_unit) or mm^3/time_unit is changing.

I am not sure how your study is setup, so I cannot really recommend anything here (e.g. when did the intervention happen exactly).

Generally I would recommend a piecewise linear model with node at the intervention time in LME (mixed effects).

Best, Martin

On 21 Apr 2017, at 17:15, Clara Kühn ckuehn@cbs.mpg.de wrote:

Dear FreeSurfer experts,

I have a longitudinal design with 3 groups and 3 time points. For one group the intervention did not work so for now I would like to look at 2 groups only. Can I do this with the Longitudinal Two Stage Model by looking at the spc from tp1 to tp2 and then looking at the spc from tp2 to tp3?

Are there statistical reasons that forbid this procedure of comparing the change across time between groups?

Thanks for your help! Clara

-- Clara Kühn, Phd Student

Max-Planck-Institute for Human Cognitive and Brain Science Department of Neuropsychology Stephanstrasse 1A 04103 Leipzig, Germany

Phone: +49 341 - 9940 2271 Fax: +49 341 - 9940 2260 Web: www.cbs.mpg.de E-Mail: ckuehn@cbs.mpg.de

---

Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer

Hi Martin,

thanks for your reply. From your email I understand that looking at the rate of change between time points is more useful than spc if I want to look at only 2 time points at a time.

I have a pre training scan, a post training scan (approximately 3 weeks apart with a training intervention in between scans) and a follow up scan after 3 weeks of no intervention for either group. Therefore I could compare the rate of change in thickness and area from pre scan to post scan between the training group and the control group. Similarly, I could check if the rates differ between the groups when looking at post scan and follow up. And that I could do with the longitudinal two stage model, right?

Could you explain what you mean with "node at the intervention time" with regards to LMEs? Also I'm not sure what you mean with piecewise. I've tried to follow the LME tutorial once before. Do you mean that I should only try the linear version of the model? How would I do this piecewise?

Thanks for your help! Clara

----- Ursprüngliche Mail ----- Von: "mreuter" mreuter@nmr.mgh.harvard.edu An: "Freesurfer support list" freesurfer@nmr.mgh.harvard.edu Gesendet: Freitag, 21. April 2017 22:09:14 Betreff: Re: [Freesurfer] REPOST: Two stage model question

Hi Clara,

the SPC (or any percent change) could be problematic, as the base value (either the volume at the baseline timepoint, or the average volume) will change depending wich interval you look at. You could check if the rate (mm/time_unit) or mm^3/time_unit is changing.

I am not sure how your study is setup, so I cannot really recommend anything here (e.g. when did the intervention happen exactly).

Generally I would recommend a piecewise linear model with node at the intervention time in LME (mixed effects).

Best, Martin

On 21 Apr 2017, at 17:15, Clara Kühn ckuehn@cbs.mpg.de wrote:

Dear FreeSurfer experts,

I have a longitudinal design with 3 groups and 3 time points. For one group the intervention did not work so for now I would like to look at 2 groups only. Can I do this with the Longitudinal Two Stage Model by looking at the spc from tp1 to tp2 and then looking at the spc from tp2 to tp3?

Are there statistical reasons that forbid this procedure of comparing the change across time between groups?

Thanks for your help! Clara

-- Clara Kühn, Phd Student

Max-Planck-Institute for Human Cognitive and Brain Science Department of Neuropsychology Stephanstrasse 1A 04103 Leipzig, Germany

Phone: +49 341 - 9940 2271 Fax: +49 341 - 9940 2260 Web: www.cbs.mpg.de E-Mail: ckuehn@cbs.mpg.de

---

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