To continue with the other contrasts
On Tue, 11 Sep 2007, Nicholas Rule wrote:
> I can test both of the within-subjects contrasts:
> [1 -1 1 -1]
> [-1 1 -1 1]
>
> I can test both of the interaction's contrasts:
> [1 -1 -1 1]
> [-1 1 1 -1]
This is because here in both cases differences are taken within subject,
and therefore subject means are not affecting the contrast.
> I can NOT test either of the between-subjects contratsts:
> [1 1 -1 -1]
> [-1 -1 1 1]
Here you have to weight the subject means by the sum of weights put on the
condition columns per group, i.e.
2*1/N1*ones(1,N1) 2*1/N2*ones(1,N2) 1 1 -1 -1
would work.
Volkmar
> the first 19 columns are all weighted with 0's (preceding the 4 contrast
> weights, above).
>
>
> Markus Lonsdale also posted recently with a similar problem in a PET study,
> I've copied his post below (sorry I couldn't figure out how to paste the URL
> without it going through my username):
>
> ----------------------------------------
> Hi
> we try to analyse a PET study with 2 groups of subjects in 2
> conditions. The idea is to look at group differences in each condition
> as well as in the response to the conditions.
>
> We have tried to model this as flexible factorial, 2 factors
> (subjects, conditions), 2 main effects (subjects, conditions) with an
> interaction between the two. (The idea is to define a contrast where
> we can "pick out" the subjects according to their group.)
>
> However, no contrasts are accepted by the contrast manager, not even
> the simple contrast "condition1 - condition2"!?!
>
> Any help is appreciated ...
>
> Markus
> ----------------------------------------
>
> Thanks,
> Nick
>
> Quoting FATIMA HUSAIN <[log in to unmask]>:
>
>>
>> To second Nick Rule's post, it would be great if someone had a solution
>> for this problem.
>> I am attaching my design matrix. I can set up the following t contrast
>> zeros(1,24) 0 0 1 -1 (thus testing within subject factor stim1>stim2)
>>
>> but I cannot set up
>>
>> zeros(1,24) 1 -1 0 0 (between subject factor grp1>grp2)
>>
>> the answer always is "invalid contrast"
>>
>> (I am using flexible factorial, three factors, with "subject" as first
>> factor, group as second factor, stimulus as third factor, and testing for
>> main effect of the three factors).
>> I tried using Full factorial method but the full factorial method does not
>> treat subject as repeated measure.
>>
>> Fatima Husain
>
>
--
Volkmar Glauche
-
Department of Neurology [log in to unmask]
Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
Breisacher Str. 64 Fax 49(0)761-270-5416
79106 Freiburg http://fbi.uniklinik-freiburg.de/
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