If you are looking for the difference in the correlation between the
groups then you have to use DODS and 0 0 1 -1. The way to understand it
is that you have four regression coefficients and each means something.
The way you have it set up in the fsgd file is that they are
1. group 1 intercept
2. group 2 intercept
3. group 1 cognition slope
4. group 2 coginition slope
Each coefficient gets multipled by its corresponding weight in the
contrast and then the results are summed. So if you use 0 0 1 -1 you end
up with contrast = g1cogslope-g2cogslope which is the difference you want
If you are just interested in whether the cog slope changes without
regard to the group, then you would use 0 0 0.5 0.5 which means contrast
= 0.5*g1cogslope + 0.5*g2cogslope which is the average of the slopes. In
this case, using DOSS and 0 0 1 will give you the same results if there
is no difference in the cog slopes between the groups
On 1/12/2023 8:48 AM, Agurne Sampedro Calvete wrote:
External Email - Use Caution
Dear FreeSurfer experts,
I have two groups and two time points and I would like to do a
correlation analysis between a variable (cognition) and the change
score in cortical thickness between the two time points (time2 minus
time1).
I would like to see if the *correlation *is *different *among each
group, if there is an interaction with the group.
I have followed the paired t test instructions (*MailScanner has
detected a possible fraud attempt from "secure-web.cisco.com" claiming
to be* https://surfer.nmr.mgh.harvard.edu/fswiki/PairedAnalysis
<https://secure-web.cisco.com/11YHUupGYyGBe5w6QAQb3OOLcNluK0Tqv4VUmAmDx5wuogvWlaypoD3OpvEWBM2c4Wa29T38k5k4_bq_JXA-kc8P0mACbCulAMmVSDKvC4Tl7x6Gk2A_azFExScL0y5HFngabH5bZRoIDy_dKvAtXkXegvTr-2ftNMkD3TQlTwaQtCSI-0554BriqrUu1MKXy2ZDf8l6cSP5im59p93FqJma0yYa3cYXujDp-aXCIOKlUXzojlgBIskwrWMiPG0pmIn08tOj1OYgSExrdJrOrWu2iAIGdhvSK3iuWEQ7BCEp4De-DPfIldNoZfbygVQ1mrt9CvPacDWwenDkjC7Q40A/https%3A%2F%2Fsurfer.nmr.mgh.harvard.edu%2Ffswiki%2FPairedAnalysis>),
but I am not sure about the contrast(s) I have to use.
The paired-diff.fsgd file (second fsgd file) is the following:
GroupDescriptorFile 1
Title OSGM
Class Group1
Class Group2
Variables cognition
Input subject1pair Group1 0.546
Input subject1pair Group2 -0.243
...
How should it be the contrasts files to see the *correlation *of the
variable cognition with the change score in thickness taking into
account the groups? And should I use DODS or DOSS?
Using DODS:
0 0 0.5 0.5
or
0 0 1 -1
I don't understand the difference between both contrasts. I think I
should use the one of "0 0 1 -1" to see whether the cognition variable
correlates with change in thickness differently according to the
group. Is this correct?
Using DOSS:
0 0 1
Thank you very much in advance.
Kind regards,
Agurne
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