Hi Mark and Jay,
there is no good answer to this question I'm afraid.
Say you've got two subjects in group A, 3 in B and 4 in C, then both approaches are valid:
Method I
A B
1 0
1 0
0 1
0 1
0 1
for the design.mat of the first A and B groups with
1 -1 (A-B)
-1 1 (B-A)
for the design.con
and then repeat for groups B and C, then groups A and C (which is what you did Mark).
Method II
A B C
1 0 0
1 0 0
0 1 0
0 1 0
0 1 0
0 0 1
0 0 1
0 0 1
0 0 1
for the design.mat of the 3 groups with
1 -1 0 (A-B)
-1 1 0 (B-A)
0 1 -1 (B-C)
0 -1 1 (C-B)
1 0 -1 (A-C)
-1 0 1 (C-A)
for the design.con (t-tests)
and
1 0 1 0 0 0
for the design.fts (F-test, as many columns as there are rows in your design.con, you just need to click in "F-tests" in the Glm gui and then click in front of the two relevant "Contrasts" you have already set up)
So with Method II, you can also ask the question of where are the changes *across the 3 groups* (F-test with the design.fts). You also get an increase in DoF but, as Tom Nichols said, if it happens that group C for instance has wildly smaller variance, you can get inflated significances (or reduced power if it has wildly larger variance).
So it depends on what your main question is, really.
Hope this helps,
Gwenaelle
--- En date de : Lun 12.4.10, Mark Walterfang <[log in to unmask]> a écrit :
> De: Mark Walterfang <[log in to unmask]>
> Objet: Re: [FSL] FSLVBM GLM Setup
> À: [log in to unmask]
> Date: Lundi 12 avril 2010, 14h18
> Hi all
>
> I'm in the same situation as Jay. I have three groups
> (illness1, illness2
> and controls), all matched to each other. I've run three
> two-way analyses,
> which is pretty laborious and I'm pretty sure it's not
> statistically ideal.
> What I can't work out is how to set up the design matrices
> & contrasts in
> the way Jay describes, as the online manual for Randomise
> doesn't really
> provide guidance here. Gwenaëlle, is this something you
> can advise on?
>
> Thanks in advance,
>
> Mark Walterfang
>
>
> On 10/4/10 12:29 PM, "Jay Ives" <[log in to unmask]>
> wrote:
>
> > I have 70 subjects in 4 groups and would like to test
> between individual
> > groups and combinations of the groups. Can someone
> please advise me how to set
> > up the design.mat and design.con files to do this?
> Thx
>
>
> WARNING: This message
> > originated from outside the Northern/Melbourne/Western
> Health e-mail network.
> > The sender cannot be validated. Caution is advised.
> Contact IT Services (+61 3
> > ) 9342 8888 for more information.
>
|