Thank you Steve.
martin
Steve Smith wrote:
> Hi,
>
> On 10 Apr 2007, at 20:46, Martin M Monti wrote:
>
>> Dear FSLers,
>>
>> In a first-level FEAT analysis I have the choice of breaking down one
>> of my tasks (it's a slow event related paradigm) into 2 different EV
>> as opposed to keeping all in 1 EV.
>>
>> So, say I have 8 trials, which can be further grouped in 2 categories
>> (4 trials each).
>>
>> I envision 2 options (at a later point I'll be interested in looking
>> at differences between the two categories, but not for now):
>>
>> a) create 1 EV file (with all 8 trials) and then create a contrast
>> with +1 in this EV only
>> b) create 2 EV files (with 4 trials each) and then create a contrast
>> with + 1 in both EVs
>>
>> Would (a) and (b) yield the same results (I'm thinking esp of the
>> variance of estimation in the voxel-wise regression)?
>
> In general they will be similar. b) reduces the degrees-of-freedom
> slightly, which reduces zstats, but also reduces the unmodelled
> variance, which increases zstats. In general the latter factor would
> be expected to win out.
>
> Cheers.
>
>
>>
>> cheers
>>
>> martin
>>
>> ----------------------------------------------
>>
>> “Well! I’ve often seen a cat without a grin,” thought Alice, “but a
>> grin without a cat! It’s the most curious thing I ever saw in my
>> life!” [Lewis Carroll, Alice in Wonderland]
>>
>> --------------------------------------------
>> Martin M. Monti
>> Princeton University
>> Department of Psychology
>> Green Hall 3-S-8
>> (609) 258-5679
>> www.webmartin.net
>> --------------------------------------------
>
>
> ---------------------------------------------------------------------------
>
> 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
> ---------------------------------------------------------------------------
>
--
--------------------------------------------
“Well! I’ve often seen a cat without a grin,” thought Alice, “but a grin without a cat! It’s the most curious thing I ever saw in my life!” [Lewis Carroll, Alice in Wonderland]
--------------------------------------------
Martin M. Monti
Princeton University
Department of Psychology
Green Hall 3-S-8
(609) 258-5679
www.webmartin.net
--------------------------------------------
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