Hi again Gwenaëlle

Many thanks, that's most helpful. Just finally - if you were a reviewer and
saw just the three two-way comparisons, would you ask for the three-group
analysis and post-hoc t-tests, or would you be happy with the three two-way
analyses?

Rgds

Mark


On 13/4/10 9:01 PM, "Gwenaëlle DOUAUD" <[log in to unmask]> wrote:

> 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
> Neuropsychiatry Unit
> Level 2, John Cade Building
> ROYAL
> MELBOURNE HOSPITAL 3050 AUSTRALIA
> T +61-3-93428750
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> E
> [log in to unmask]
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> 
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> Fellow
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> & 3, Allan Gilbert Building
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> +61-3-83441800 
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> 





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--
Dr Mark Walterfang
Consultant Neuropsychiatrist
Neuropsychiatry Unit
Level 2, John Cade Building
ROYAL MELBOURNE HOSPITAL 3050 AUSTRALIA
T +61-3-93428750
F +61-3-93428483
E [log in to unmask]
W www.neuropsychiatry.org.au
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