Hi Douglas,

Thank you so much for your response. We just want to be extra careful about the way we are doing this and so below I have outlined the exact pattern of the group descriptor files we are using along with the contrast. Please let me know if you think this is correct or not. The questions we are asking with this analysis are the same as I outlined in the previous email. 

This is our first group descriptor file:

GroupDescriptorFile 1

Title BEIP 8 vs. 16 compare CAU (0) And FCG (1)

Class Main

Input    005_16.long.005    Main

Input    005_8.long.005    Main

Input    006_16.long.006    Main

Input    006_8.long.006    Main

This is our second group descriptor file:

GroupDescriptorFile 1

Title BEIP longitudinal Compare CAU (0) And FCG (1)

Class cau

Class fc

Input    005_long_pair fc

Input    006_long_pair cau

The contrast to understand group differences in thickness between 8 and 16 yr old data sets are:

1 -1

Question1: Is this correct?

Question 2: If we use the below mentioned way to set up our second group descriptor file, will the above contrasts still work? Or can we use 0 1 contrast with the file below to get at the same thing? We actually ran it both ways and the results weren't similar but I specifically ask this question because we are a little worried about the above 'second group descriptor file' not having a column that says 'Main' and we are trying to understand the right way to do our analyses.

Example group descriptor file 2:

GroupDescriptorFile 1

Title BEIP longitudinal Compare CAU (0) And FCG (1)

Class Main

Variables group

Input    005_long_pair Main 1

Input    006_long_pair Main 0

Thank you so much! I appreciate all your help with this!

Best,

Srishti

Social/Clinical Research Specialist

Child Imaging Research and Life Experiences Lab

University of North Carolina at Chapel Hill

email (W): srishtig@email.unc.edu

skype: srishti.goel12

On Wed, Nov 7, 2018 at 9:24 AM Greve, Douglas N.,Ph.D. <DGREVE@mgh.harvard.edu> wrote:

On 10/24/2018 09:45 PM, srishti goel wrote:
>
>         External Email - Use Caution
>
> Hello,
>
> I am trying to perform a paired analysis for a data set that has
> subjects' who belong to either group1 or group2 and have data at time
> point1 (tp1) and time point2 (tp2). I am following all the steps on
> the wiki but I want to clarify if the contrasts I plan to use for my
> analyses are appropriate.
>
> For my analyses, I only have two groups and no covariates. I am
> interested to know if the decrease in cortical thickness of subjects
> in group 1 across tp1 and tp2 is more than the decrease in cortical
> thickness of subjects in group 2 across tp1 and tp2. The contrast I
> think should be used for this is: 1 -1. Is that correct?
Yes, if you first take the difference between tp1 and tp2.
>
> I am also interested to know if the cortical thickness decreased for
> subjects regardless of the groups so should I use the contrast: 1 for
> that?
Assuming you are keeping 2 groups, then use
  0.5 0.5
>
> Best,
> Srishti
> Social/Clinical Research Specialist
> Child Imaging Research and Life Experiences Lab
> University of North Carolina at Chapel Hill
> email (W): srishtig@email.unc.edu <mailto:srishtig@email.unc.edu>
> skype: srishti.goel12
>
>
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