Dear scientists,

I have a question about how to compare mean for two different groups by using linear mixed model.

Each mask nests each braintract, brain tract nest each individiual. (Many people in these two groups, only show two people's brain structure for illustration)
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So since the FA value of each mask in the same braintract, instead of using two sample t-test or ANOVA, I fit nest linear mixed model as this
Code:
mixed fa group i.sex age || id: || braintract: , vce(robust) cov(un)
The coefficients of the output regression table only show the effect that only compare the first level.
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1, My question is how can I compare different FA mean between two groups for each mask.
I expect to get the comparison of FA mean for two group
Group A v.s. Group N
Mask1braintract1 v.s. Mask1braintract1
Mask1braintract2 v.s. Mask1braintract2
Mask1braintract3 v.s. Mask1braintract3
Mask2braintract1 v.s. Mask2braintract1
Mask2braintract2 v.s. Mask2braintract2
Mask2braintract3 v.s. Mask2braintract3
Mask3braintract1 v.s. Mask3braintract1
Mask3braintract2 v.s. Mask3braintract2
Mask3braintract3 v.s. Mask3braintract3

2, Another question is, since mask nest in braintract, braintract nest in individual, I already put id and braintract as random effect, should I consider mask as random effect like this?
Code:
||id: || braintract: || mask
Below is example of my data structure

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Thank you so much for looking at my post!

Jack Liang