Hi - this is pretty much right, though it does include cross-session
and cross-subject variance within one analysis, which means that if
you intend this to be a cross-subject mixed-effects analysis (which
you probably do) then you are overestimating the degrees-of-freedom
in this analysis and your results will be over-liberal. Ideall, you
should move the cross-session analyses into a second-level analysis,
and bring single image summaries for each condition of each subject
up into the quadrupled-t-test-across-subjects for a third-level
analysis (ie a combination of what you have here and the three-level
example in the manual).
Cheers, Steve.
On 28 Nov 2006, at 19:05, Stephane Jacobs wrote:
> Hello everyone,
>
>
> Sorry to bug you with the details of my analysis, but I would need
> to check
> that what I'm doing is correct... I have of course looked up the
> archives,
> but did not find the answers to ALL my questions.
>
> I have 16 subjects, each scanned under 4 conditions, 2 or 3 runs per
> condition (depending on the subject). I would like to contrast these
> conditions with each other. I therefore used an extended version of
> the
> Tripled T-test described in the FEAT manual, with a design as
> follows (with
> only 2 subjects, 3 runs per condition, for the sake of clarity):
>
> EV1 EV2 EV3 EV4 EV5
> S1 – Ar1 1 1 1 1 0
> S1 – Ar2 1 1 1 1 0
> S1 – Ar3 1 1 1 1 0
> S1 – Br1 -1 0 0 1 0
> S1 – Br2 -1 0 0 1 0
> S1 – Br3 -1 0 0 1 0
> S1 – Cr1 0 -1 0 1 0
> S1 – Cr2 0 -1 0 1 0
> S1 – Cr3 0 -1 0 1 0
> S1 – Dr1 0 0 -1 1 0
> S1 – Dr2 0 0 -1 1 0
> S1 – Dr3 0 0 -1 1 0
> S2 – Ar1 1 1 1 0 1
> S2 – Ar2 1 1 1 0 1
> S2 – Ar3 1 1 1 0 1
> S2 – Br1 -1 0 0 0 1
> S2 – Br2 -1 0 0 0 1
> S2 – Br3 -1 0 0 0 1
> S2 – Cr1 0 -1 0 0 1
> S2 – Cr2 0 -1 0 0 1
> S2 – Cr3 0 -1 0 0 1
> S2 – Dr1 0 0 -1 0 1
> S2 – Dr2 0 0 -1 0 1
> S2 – DR3 0 0 -1 0 1
>
>
> And I have set the contrasts as following, assuming that the weight
> for each
> contrasts would be A = a + b + c; B = -a; C = -b; D = -c (a for
> Ev1, b for
> EV2 and c for EV3).
>
>
> EV1 EV2 EV3 EV4 EV5
> A – D 1 1 2 0 0
> D – A -1 -1 -2 0 0
> B – C -1 1 0 0 0
> C – B 1 -1 0 0 0
> A – B 2 1 1 0 0
> B – A -2 -1 -1 0 0
> D – C 0 1 -1 0 0
> C – D 0 -1 1 0 0
> AB – CD 0 2 2 0 0
> CD - AB 0 -2 -2 0 0
>
>
> So, basically my question is simple: is this correct? :-)
>
> I think it is, if I understood the Tripled T-test example, but it's
> counter-intuitive enough for me to still doubt... Especially since
> I average
> across sessions within each subject.
>
>
> Also, I will run a separate second level analysis to get the mean
> group
> effect for each condition, with an EV per condition, and an input
> per run
> within each subject. Still correct?
>
>
> Thanks a lot for all your help!
>
> Best regards,
>
> Stephane
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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