I would use GLM Flex or a one-way within-subject ANOVA since you have
4 measures per subject.
But to answer your exact question, the contrast to compare C1 to C2
would be 1 -1 for C1>C2 T-test and -1 1 for C2>C1 T-test. The F-test
can be 1 -1 or -1 1. Both will be the same. T-tests will test your
directional hypothesis, the F-test tests for either direction.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Tue, Nov 13, 2012 at 1:56 PM, Desbot Julie <[log in to unmask]> wrote:
>
>
> Dear SPMers,
>
> I have made an experiment where 20 people had to look at fearfull images vs
> neutral images.
> Each subject watched the same run 4 times, with 1 parameter varying each
> time (so in total I have 4 times for the same run). But I can’t compare the
> 4 sessions together because they differs from the previous one by a single
> parameter. For instance the 3rd session differs from the 1st one by 2
> parameters.
> To see if one parameter has an effect, I have done a paired t-test between 2
> conditions (time 1 vs time 2 ; time2 vs time 3 ; time 1 vs time 4).
> First I would like to know if the paired t-test is appropriate and second, I
> am not sure which contrast (F or T) I am supposed to use.
> Is it correct to do a F test with the vector [-1 1] (cf. picture attached to
> this mail) or do I have to complete for all the columns [-1 1 -1 1 -1 1...]
> ?
>
> Any help would be more than appreciated!
> Thank you very much in advance.
>
> Julie
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