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Following Fatima's lead, and at Ged's suggestion, here is a picture of 
my design matrix.

As you can see, I've specified a main effect for subject, producing the 
19, staggered, step-like columns. After these, there are four columns 
(the modeled interaction). The first two columns refer to the first 
level of the between-subjects effect. The last two columns refer to the 
second level of the between-subjects effect. Columns 1 and 3 refer to 
the first level of the within-subjects effect; Columns 2 and 4 refer to 
the second level of the within-subjects effect.

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]

I can NOT test either of the between-subjects contratsts:
[1 1 -1 -1]
[-1 -1 1 1]

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):

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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
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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