Dear James,
I mis-expressed myself. Actually I wanted to measure a V-shape,
because the levels are equally distant between each other. A contrast
of [2 -1 -2 -1 2] would make the difference between 1st and 2nd = 3,
while the 2nd from 3d = 1. For my purpose Jason's method is right.
Thanks a lot for the help.
Dorian.
2008/12/10 James Rowe <[log in to unmask]>:
> Dear Dorian and Jason,
>>
>> If you take the contrast:
>> c = [3 2 1 2 3]
>> and then subtract the mean you get the contrast:
>> c = c - mean(c);
>> c = [0.8 -0.2 -1.2 -0.2 0.8]
>> This contrast tests for an equal difference between each adjacent
>> condition and has a sum of zero.
>
> this approach would identify a 'V' shaped contrast. For a neurobiological
> system this might not be quite what you want.
> If you wanted to capture a 'U'-shaped function, indicative of a typical
> Yerkes-Dodson type neuropsychological or psychopharmacological function,
> then I would suggest you stick to the mean corrected quadratic instead, eg.
> [2 -1 -2 -1 2], as you originally had it.
> In practice, for five levels, the corelation coefficient between [2 -1
> -2 -1 2] and [0.8 -0.2 -1.2 -0.2 0.8] is ~0.95 , so it is unlikely
> to make much difference.
> best wishes,
> James
>
>
>
>
>
>
>>
>>
>> I want to catch the brain areas that are activated like a U-shape in 5
>> conditions. In practice the two extremes would have a value of 1, and
>> the one in the midle -1, However the two remaining values would be
>> -0.5 each:
>>
>> [1 -0.5 -1 -0.5 1]
>>
>> However this doesn't reflect the change I want to catch, because the
>> second condition changes from the first one much more (1.5) then the
>> second from the third (0.5).
>>
>> Anybody had this problem before? What numbers would catch the signal
>> change in the attached example?
>>
>> Regards.
>> Dorian
>> Ruhr University of Bochum
>>
>>
>
>
> --
>
> --------------------------------------
> Dr James Rowe
> Senior Clinical Research Associate and
> Consultant Neurologist, Cambridge University Department of Clinical
> Neurosciences,
> Box 83, R3 Neurosciences,
> Addenbrooke's Hospital, Cambridge, CB2 2QQ
> UK
>
> Tel: +44 (0)1223 273630
>
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