Hello,

this is somewhat difficult to answer, so the following is a personal
opinion.

I think the first question is whether your would like to go for a) a
cross-sectional design or b) a longitudinal design.

If a), you'd have a single scan per subject (e.g., baseline) and could
discard the temporal information. So if you do not have a longitudinal
research question and your goal is to model group differences at a
single time-point, it is not necessary (and overly complicated,
actually) to use a LME model.

If b), you'd have multiple scans per subject, and would therefore use
the LME model. I would not recommend to exclude the timing information
from the model / design matrix in this case, no matter how close the
temporal spacing may be. It is still possible to test for effects other
than time, e.g. for group differences at baseline.

Best regards,

Kersten

On Mi, 2018-04-04 at 23:37 +0200, Kaushal, Mayank wrote:
> The continuous variable in my analysis is time_base_scan with the
> time points evaluated by me being of acute and subacute nature.
> However, due to the small differences in time elapsed between
> successive visits (duration between visits is a couple of weeks), is
> it possible to simply model LME analysis around categorical variables
> without including continuous variables?
> 
> More specifically, I want to see the effect of group. Further, I
> don’t intend to include time_base_scan as a continuous variable.
> What would the contrasts used by me to evaluate a categorial
> variable?
> 
> Further, my analysis includes three groups. So, my understanding is
> that evaluating group as a categorical variable would’ve no bearing
> on the column composition of X and they would remain the same as
> mentioned in my previous mail.
> 
> The columns in X are as follows:
> Column 1: intercept
> Column 2: time_base_scan (This signifies the time elapsed from the
> base scan in days. Each subject had upto 4 visits with scans
>                  undertaken on each visit. For the scan taken on the
> first visit 0 was entered in the qdec.dat.table file)
> Column 3: age1stscan (This is the age of the subject at first scan)
> Column 4: group 1 (Value of 1 entered in this column if the subject
> belonged to group 1 and value of 0 entered in column 5 that signifies
> group 2)
> Column 5: group 2 (Value of 1 entered in this column if the subject
> belonged to group 2  and value of 0 entered in column 4 that
> signifies group 1)
> 
> Mayank
> On Mar 30, 2018, at 6:30 AM, Diers, Kersten /DZNE <Kersten.Diers@dzne
> .de<mailto:kersten.di...@dzne.de>> wrote:
> 
> ATTENTION: This email originated from a sender outside of MCW. Use
> caution when clicking on links or opening attachments.
> ________________________________
> Hello,
> 
> please find my responses below.
> 
> Best regards,
> 
> Kersten
> 
> On Do, 2018-03-29 at 00:11 +0200, Kaushal, Mayank wrote:
> 
> Hi Kersten,
> 
> Apologies for the delay. I am still in the process of trying to
> figure out the gaps in my understanding and would appreciate your
> inputs.
> 
> I created the design X from M using: X = [ones(length(M),1) M
> M(:,1).*M(:,3) M(:,1).*M(:,4)];
> 
> The columns in X are as follows:
> Column 1: intercept
> Column 2: time_base_scan (This signifies the time elapsed from the
> base scan in days. Each subject had upto 4 visits with scans
>                   undertaken on each visit. For the scan taken on the
> first visit 0 was entered in the qdec.dat.table file)
> Column 3: age1stscan (This is the age of the subject at first scan)
> Column 4: group 1 (Value of 1 entered in this column if the subject
> belonged to group 1 and value of 0 entered in column 5 that signifies
> group 2)
> Column 5: group 2 (Value of 1 entered in this column if the subject
> belonged to group 2  and value of 0 entered in column 4 that
> signifies group 1)
> Column 6: Column 2 (time_base_scan) x Column 4 (group 1)
> Column 7: Column 2 (time_base_scan) x Column 4 (group 2)
> 
> So my understanding is that subjects belonging to group 3 have 0 in
> both columns 4 and 5 and consequently, columns 6 and 7 would also be
> 0 for them. This would be translated by matlab lme toolbox while
> doing spatiotemporal analysis. Is my understanding correct?
> 
> 
> Yes, this is correct, apart probably from a little typo:
> 
> Column 7 should read: " Column 2 (time_base_scan) x Column 5 (group
> 2)"
> (group2 is column 5, not 4, in X; please adapt your design matrix if
> necessary)
> 
> Group 3 will be your reference group then, and will implicitly
> modeled by this design matrix. I speculate this is what you mean by
> 'translated'.
> 
> 
> I have attached X as well as M as separate excel files for your
> consideration.
> In addition, I have attached qdec.dat.table file as well a word doc
> labeled “workflow” that detail the order of matlab functions I have
> used to perform the analysis.
> 
> 
> 
> My objective is to highlight clusters that are significantly
> different between the groups over time using spatiotemporal analysis.
> 
> The contrasts used by me for the analysis: CM.C = [0 0 0 0 0 1 0; 0 0
> 0 0 0 -1 1]
> 
> Kindly comment on my choice of contrasts. Is this the correct choice
> for contrasts if I want to find significant clusters based on
> cortical thickness between all three groups over time?
> 
> 
> 
> Yes, the contrast is also correct for your purpose. It will identify
> regions in which cortical thickness changes across time differ
> between groups.
> 
> 
> Mayank
> 
> 
> _______________________________________________
> Freesurfer mailing list
> Freesurfer@nmr.mgh.harvard.edu<mailto:Freesurfer@nmr.mgh.harvard.edu>
> https://urldefense.proofpoint.com/v2/url?u=https-3A__mail.nmr.mgh.har
> vard.edu_mailman_listinfo_freesurfer&d=DwIBAg&c=aFamLAsxMIDYjNglYHTMV
> 0iqFn3z4pVFYPQkjgspw4Y&r=RqvEwdmcEg_aWsE7PBL80w&m=jz6TYzDc4SmT6ar1_JE
> w5rF4JCOlAxM5KmH1GK_SDNc&s=9INo1YCHATevCPV_UsV08YkkiXAOEcUSw7nvJLvyBa
> g&e=
> 
> 
> The information in this e-mail is intended only for the person to
> whom it is
> addressed. If you believe this e-mail was sent to you in error and
> the e-mail
> contains patient information, please contact the Partners Compliance
> HelpLine at
> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.partners.org_
> complianceline&d=DwIBAg&c=aFamLAsxMIDYjNglYHTMV0iqFn3z4pVFYPQkjgspw4Y
> &r=RqvEwdmcEg_aWsE7PBL80w&m=jz6TYzDc4SmT6ar1_JEw5rF4JCOlAxM5KmH1GK_SD
> Nc&s=guYHSBVwZaPkOne0blcYjP_gI5nw3ZybG4nnnHojJKk&e= . If the e-mail
> was sent to you in error
> but does not contain patient information, please contact the sender
> and properly
> dispose of the e-mail.
> 
> 
> _______________________________________________
> Freesurfer mailing list
> Freesurfer@nmr.mgh.harvard.edu
> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
> 
> 
> The information in this e-mail is intended only for the person to
> whom it is
> addressed. If you believe this e-mail was sent to you in error and
> the e-mail
> contains patient information, please contact the Partners Compliance
> HelpLine at
> http://www.partners.org/complianceline . If the e-mail was sent to
> you in error
> but does not contain patient information, please contact the sender
> and properly
> dispose of the e-mail.

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

Reply via email to