External Email - Use Caution Hi Bruce,
Thanks for the prompt reply. It is my understanding that l_thickness, r_thickness and estimated intracranial volume are accurately measured. Would I still need to identify and remove outliers if visual inspection of the images has not been done? Many thanks, Stephanie On Wed, Nov 18, 2020 at 5:24 PM Stephanie K <rklin...@gmail.com> wrote: > Hi, > I want to estimate the mean cortical thickness. For this I have summed the > thickness across all 34 regions mapped to the Desikan-Killiany atlas. > However, I also have the average mean thickness of left and right > hemispheres (direct output variables of Freesurfer). As there is no visual > inspection of the imaging in the particular cohort, I remove measures that > are 3 standard deviations above or below the mean. Hence, I may expect more > outliers to be removed when I take the average across the regions. I am > using these brain measures as outcomes in association analyses with the > genetic score as the exposure. For the mean thickness (averaged across the > left and right hemisphere thickness variables of freesurfer after removing > outliers), the regression coefficients have a smaller standard deviation > than with thickness averaged across the 34 regions. I’m not sure which one > to use - which one is more accurate? When I look at the mean thickness > (which I derived using 34 regions) and it’s standard deviation, it is > similar to that of the average mean thickness across the two hemispheres as > well as the standard deviation of that. Can you suggest what is most > accurate please and what the difference is between the mean thickness > across the two hemispheres obtained from freesurfer and those calculated > across the regions? Why does one result in more precision than in the other? > > Thank you! >
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