Hi Gwenaëlle
Thanks for your response - I guess I am interested in the result of an
F-test across the three samples so it sounds like it will be worth a try.
With regards to what my question is: it's partially answered by my results
from the three 2-way analyses. I see that illness1 vs controls shows some
key reductions; illness2 vs controls shows more widespread reductions and in
larger clusters; illness1 vs illness2 shows no differences in either
direction. I'm interested in whether illness1 vs illness2 really differ, as
the separate comparisons against controls implies that they should, but a
direct comparison between them suggests that they don't. (My numbers are
20-30 in each group). Does method II assist in this?
Rgds
Mark
On 13/4/10 4:20 AM, "Gwenaëlle DOUAUD" <[log in to unmask]> wrote:
> 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
>
>
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Dr Mark Walterfang
Consultant Neuropsychiatrist
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