Thanks Steve.
Yes, but in this case, A,B, and C are levels of task difficulty
actually.
Not just three sessions of the same thing.
I was interested in mean task activity in the absence of the
difficulty modulation.
That's a little different than just using the fixed effects option
isn't it?
-Brad
On Nov 25, 2008, at 1:48 AM, Steve Smith wrote:
> Hi,
>
> A = a + b + c
> B = a -b
> C = a-c
>
> A+B+C = 3a
>
> So yes a [1 0 0 ] contrast gives you the mean of A,B,C.
>
> However if you just want to ignore cross-session variability you
> could just use an all 1s single EV and use the fixed-effects option?
>
> Cheers.
>
>
>
>
> On 24 Nov 2008, at 22:04, Brad Goodyear wrote:
>
>> Hi.
>> I want to compute the average across three conditions for each
>> subject by removing any
>> differences between the conditions.
>> For two conditions, I understand the EVs would be
>>
>> EV1 EV2
>> 1 1
>> 1 -1
>>
>> would it not, and I compute the contrast (1,0) for the average
>> across the two conditions
>> with any differences removed?
>>
>> For three conditions, is it
>>
>> EV1 EV2 (a) EV3 (b)
>> 1 1 1
>> 1 -1 0
>> 1 0 -1
>>
>> and then compute (1,0,0) since conditions A+B+C = a+b-a-b = 0, as
>> per the tripled t-test
>> example?
>> I then plan to average this (1,0,0) contrast across subjects to
>> get the mean across
>> condition with any differences between the conditions removed.
>>
>> -Brad
>>
>
>
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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)
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