Hi Julian,

The within-subject variability is usually much smaller than the between-subject variability such that, although there can be differences in results (with more power where these lower-level are considered), usually we don't see massive differences (as in "robust activations" you mention). FEAT doesn't see the between-subjects dependencies in the case of twins/relatives, and if there are many such pairs in the sample, false positives will be inflated (this isn't a problem with FEAT, it's just how the GLM works, and would happen even in a permutation test that didn't consider these dependencies).

All the best,

Anderson

On Fri, 26 Oct 2018 at 09:16, Julian Macoveanu <[log in to unmask]> wrote:

Dear Tom,

Thanks for the reply. Actually I already did it in PALM with correct
exchangeability block file, but I get nothing significant, not even
across all subjects. My understanding is that one benefit of FLAME
estimation is that it accounts for within-subject variability and palm
does not since only the cope files from the 1st level are used. Maybe
that could be the reason I got robust activations in FLAME but not
PALM.

Julian
On Fri, Oct 26, 2018 at 2:11 PM Thomas Nichols
<[log in to unmask]> wrote:
>
> Dear Julian,
>
>> Just wanted to confirm that I got the GLM right. I got 3 groups of
>> subjects: low-risk, high-risk & affected. The participants are twins,
>> most but not all have both twins present. The twin pairs can be in the
>> same group i.e. the low-risk group, but can also be split between the
>> high-risk and affected groups.
>>
>> I have 3 EVs for the three groups and in order to account for
>> within-pair variance correlation I have an extra EV for each twin pair
>> with two 1s for the respective twins pair and 0 otherwise. Would this
>> be an accurate GLM setup?
>
>
> Yes, this approach is 'safe' in that the residuals will be independent. It's not optimal, though, as you are effective have to toss out any variance that is common to each twin pair.
>
> If you did this in PALM, you could model this as a simple 1-way ANOVA, and then tell PALM about the twin structure. But I need to defer to Anderson to tell you how to hand-craft the exchangeability block file. Anderson?
>
> -Tom
>
>
>>
>> Cheers,
>> Julian
>>
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>
>
> --
> __________________________________________________________
> Thomas Nichols, PhD
> Professor of Neuroimaging Statistics
> Nuffield Department of Population Health | University of Oxford
> Big Data Institute | Li Ka Shing Centre for Health Information and Discovery
> Old Road Campus | Headington | Oxford | OX3 7LF | United Kingdom
> T: +44 1865 743590 | E: [log in to unmask]
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