Hi Mark,
> 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.
Sure, but bear in mind that it will only tell you where are the significant changes across the 3 groups, so you will still need to run the post-hoc t-tests on each pair of groups to determine what's "driving" these results.
> 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.
Yes, it can happen, this means that though illness2 seems more severe, this is not a significant effect.
> (My numbers are
> 20-30 in each group). Does method II assist in this?
There is no way of knowing for sure, and if it does when you'll do the post-hoc t-test between illness1 and 2, this would be just because you have increased your DoFs, not because you would have asked a different question...
Cheers,
Gwenaelle
> 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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>
>
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>
> Dr Mark Walterfang
> Consultant Neuropsychiatrist
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>
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