Hi Matthieu,
replacing time by score is very different from adding score as a
covariate. Often scores are crude and often they are constant in
controls (always full score), and only vary slightly in diseased. In
those cases it may not be good to use score as a time variable.
I would either add avg. score as a variable (fixed across time - would
probably do this for intial testing anyway) or score as time-varying
covariate, but would read up on how to interpret results in the presence
of time-varying-covariates. I am not a statistician.
Best, Martin
On 05/16/2017 08:09 PM, Matthieu Vanhoutte wrote:
Hi Martin,
Thank you for this detailed answer.
Are replacing time by score or include score as time-varying covariate
leading to the same result because of looking at the same effect ?
My willing would be to find patterns of atrophy rate/progression
correlated with cognitive score. In context of AD which method would
be the best for you ?
Best,
Matthieu
Le 16 mai 2017 11:17 AM, "Martin Reuter" <mreu...@nmr.mgh.harvard.edu
<mailto:mreu...@nmr.mgh.harvard.edu>> a écrit :
Hi Matthieu,
one option is to replace time with score in the model. That should
be straight forward.
The other option is to include score as a time-varying covariate.
If your score is not varying much across time and you are more
interested if the average score (or baseline score) affects
atrophy rates, you can also include is as a standard (fixed across
time) co-variate (such as baseline age) with a time (and a group
etc) interaction.
Sorry, but I cannot do your design. Ultimately the model is the
research question that you are asking and it is important that
this is done correctly. Maybe there is a local biostats person
that you can talk to?
Best, Martin
On 05/15/2017 08:20 PM, Matthieu Vanhoutte wrote:
Hi Martin,
Thank you. How should this variable be coded ? Should it be as
age covariate where age at baseline is used along all time points
of each subject ?
Could you provide me an example of design matrix, I don't manage
to see what does it look like to.
Best regards,
Matthieu
Le 14 mai 2017 8:08 PM, "Martin Reuter"
<mreu...@nmr.mgh.harvard.edu
<mailto:mreu...@nmr.mgh.harvard.edu>> a écrit :
Hi Matthieu,
yes, that is possible. Instead of group, you use a variable
for your score (and interaction etc). Sometimes it may also
makes sense to use score instead of time.
Best, Martin
> On 12 May 2017, at 10:51, Matthieu Vanhoutte
<matthieuvanhou...@gmail.com
<mailto:matthieuvanhou...@gmail.com>> wrote:
>
> Dear Freesurfer's experts,
>
> I have searched through the mailing list but haven't found
any answer to my question.
>
> Is it possible with LME model to make correlations between
for example cortical thickness surface data and cognition
scores along time ? As it is possible to test for interaction
of group X time, is this also in the same way feasible to
test for coognition score X time on cortical thickness ?
>
> Many thanks for your advice !
>
> Best regards,
> Matthieu
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