Thank you for your quickly answer Jorge.
What is the rule for decide if we have to use a quadratic term in the
model? That is, in your example you use the 80% of the length of the
lh.cortex as a threshold. Is this value an arbitrary one?
Thank you again,
A 2016-11-28 16:05, jorge luis escrigué:
> Hi Jordi
>
> You have implicitly included the reference group in your statistical
> model by including an intercept term and a time variable:
>
> group0 intercept: ß1
> group0 time: ß2
> group1 intercept: ß1+ß3
> group1 time: ß2+ß4
> group2 intercept: ß1+ß5
> group2 time: ß2+ß6
>
> From here you can quickly see that your proposed contrast for the
> differences in the mean response over time between group1 and group2
> is correct.
>
> Finally the region growing estimation method should yield more
> precise (lower variance) estimates for the ßs but I think it’s (might
> be) too complicated to setup with the current scripts. The vw method
> will produce good estimates too. So I recommend the vw estimation
> method if things get too complicated for you with the region-wise
> method.
>
> Cheers
> -Jorge
>
>> -------------------------
>> DE: Jordi Pegueroles <
jordi@pegueroles.cat>
>> PARA: Freesurfer <
freesurfer@nmr.mgh.harvard.edu>;
>>
jbernal0019@yahoo.es>> ENVIADO: Lunes 28 de noviembre de 2016 5:11
>> ASUNTO: LME: contrast questions
>>
>> Dear FS experts,
>>
>> I am trying to run a LME model for three groups (group0, group1 and
>> group2) and (almost all) with three time points, all processed using
>> the
>> Longitudinal FS stream.
>>
>> Our intention is to compare the differences between group0 against
>> group1 and group0 against group2. I have followed the online LME
>> tutorial and I have adapted your code to my data. However, I think I
>> have not well understood the steps and I have few questions that you
>> will be able to answer me...
>>
>> i) In the X matrix I put the following columns:
>>
>> 1) intercept
>> 2) time (years from baseline)
>> 3) group1 (binary variable: 1 if belongs to group1)
>> 4) group1.*time
>> 5) group2 (binary variable: 1 if belongs to group2)
>> 6) group2.*time
>>
>> I don't understand why I do not have to include the reference group
>> (in my case group0 and group0.*time) in the design matrix. How I can
>> see
>> the progresion of the reference group if I did not include it in the
>> design matrix? How I can compare both progressions if the time of the
>> group0 subjects is not taking into account?
>>
>> ii) Linked to the previous question, if I want to compare the
>> differences in time between group1 and group2... Should the contrast
>> be
>> C = [0 0 0 1 0 -1]?
>>
>> iii) Finally, between the vw method and the region growing method
>> which
>> one is more accurate? In other words, which one do you recommend to
>> use?
>>
>> Thank you,
>>
>> --
>> Jordi.
--
Jordi.