Gabriel Go.Es. wrote:

Thank you so much, Doug

I've check at your update and saw that this contrast has to be /2, wich I guess means 0.5 instead of one to keep the scale right?

Yes.

and the contrast for me, would be something like: Let's suppose group1_males group1_females group2_males group2_females group3_males group3_females for differences between males and females (males > females) regreessing the efects of group would be: [0.333 -0.333 0.333 -0.333 0.333 -0.333]

Correct

differences between group1 & group 3 regressing the effects of gender: [0.5 0.5 0 0 -0.5 -0.5]

Correct

and the differences between males and females for a single group can be: [1 -1 0 0 0 0] am I right???

Yes

but I don't yet figure out how to set a difference from 0 in any of the group means (to meassure the effect of group), nor the interaction between group and gender, and that's because my model is not like the one from the example,

Try using [1 -1 -1 1 0 0] [1 0 0 0 -1 1]

This is a single matrix with two rows. The first row tests for an interaction between G1/2 and M/F. The second row tests for G1/3 and M/F. Note that you don't need a third row.

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Date: Wed, 9 Mar 2011 14:52:37 -0500 From: greve@nmr.mgh.harvard.edu To: gabriellbk@hotmail.com CC: freesurfer@nmr.mgh.harvard.edu Subject: Re: [Freesurfer] glm question

That contrast is wrong. I've updated this page with the correct contrast https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf6G0V which is:

|+1 -1 0 -1 +1 0| |+1 0 -1 -1 0 +1|

This is an F-contrast (two rows). The first row looks for a difference between L and R for M and F (ie, a simple interaction between LR and MF). The second row looks for a difference between L and A for M and F. If either of these subcontrasts shows an effect, then there is an interaction.

doug

Gabriel Go.Es. wrote:

...

Dear FreeSurfer experts,

I'm doing some group analyses and I'm a little stocked with this:

I have Six Groups (2 Factors, one with three levels and the other one with two levels), and one covariate. I don't know the rule for

setting

...

...

the contrasts for an ANOVA and for the interaction between the two factors regressing out the covariate. I've checked at the FS wiki and found the following contrast for six groups (two factors) and zero covariates.

Contrast 6 gender-x-handedness.mtx (interaction between gender and handedness)

[0.333 -0.333 -0.333 0.333 0.000 0.000 0.000 0.000 0.000]

[0.000 0.000 0.000 0.000 0.000 0.333 -0.333 -0.333 0.333]

I don’t understand the rule that has been followed.

Can any one help me with this?

Bests,

Gabriel

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