Hi Kirsch,
Your design is similar to the example in page 7-8 of
Gitelman-Glascher's tutorial. Apparently you have to weight the
subjects as well:
ones(1,g1)/g1 -ones(1,g2)/g2 0.5 0.5 -0.5 -0.5
where g1 and g2 are the number of respective subjects in each group.
You can have a look yourself in the attached pfd.
Good luck.
Dorian
2009/10/22 Kirsch, Peter <[log in to unmask]>:
> Hi BaSti,
>
> great to hear from you and many thanks for your very helpful suggestions. After reading your posting I immediately realized that I over specified my model which makes it senseless to test the contrast I defined. However, a small problem I still have is that I cannot test for group main effects in the model you proposed. My regressors are the following (G=Group, C=Condition): G1C1 G1C2 G2C1 G2C2. I can test for condition main effects (1 -1 1 -1 and -1 1 -1 1) as well as for interactions (1 -1 -1 1 and -1 1 1 -1) but the group main effects (1 1 -1 -1 or -1 -1 1 1) do not work. I am sure it is again a very simple mistake but right now, I don't know which.
>
> Cheers,
>
> Peter
>
>
> -----Ursprüngliche Nachricht-----
> Von: [log in to unmask] [mailto:[log in to unmask]]
> Gesendet: Donnerstag, 22. Oktober 2009 12:29
> An: Kirsch, Peter
> Cc: [log in to unmask]
> Betreff: Re: [SPM] Contrast error in Flexible Factorial Design
>
> Dear Peter,
>
> if I get you right you did the following:
>
> 24 subjects overall
> 2 groups
> 2 conditions for every subject
>
> You modeled the subject factor as main effect on 2nd level.
> Then question is know how you modeled the factors group and condition.
> One good way to do this would be to model the interaction between
> group and condition.
>
> If you model your desing this way, you get 28 columns. Column 1-24 are
> the subjects. Column 25 is condition 1 for all subjects of group 1.
> Column 26 is conditon 2 for all subjects of group 1. Column 27 and 28
> are condition 1 and 2 for the subjects of group 2. If you are
> interested in condition 1 > condition 2 in group 1 your contrast
> vector would look like [zeros(1,24), 1, -1, 0, 0]. Condition 1 >
> condition 2 in group 2 would be [zeros(1,24), 0, 0, 1, -1]. These are
> simple effects, however. If you are interested in the main effect,
> your contrast vector would be [zeros(1,24), 1, -1, 1, -1].
>
> Hope that helps,
>
> BaSti
>
>
> Zitat von Peter Kirsch <[log in to unmask]>:
>
>> Dear colleagues,
>>
>> I would be very grateful if anybody would have an idea how to solve the
>> following problem:
>>
>> I set up a 3 factorial mixed effects design with 24 subjects using the
>> flexible factorial design option in SPM5. I have one wihtin-subject factor
>> (2 levels: condition) and one between-subject factor (2 levels: group). I
>> modelled the subject effect as the first factor. The design matrix was
>> estimated but when I try to calculate contrasts, SPM tells me that my
>> contrasts are invalid.
>> The first contrast is testing condition 1 > condition 2 and I defined it as
>> follows:
>> [zeros(1,24), 1, -1]
>> Any contrast with less then 25 columns is working but whenever I try to
>> define a contrast for my experimental factors, I get an error message.
>> I am quite sure that there is an answer to this in the depth of the archive
>> but unfortunately I could not find it.
>>
>> Thanks in advance for your invaluable support,
>>
>> Peter
>>
>> --
>> Peter Kirsch, PhD
>> Division for Imaging in Pychiatry
>> Central Institute of Mental Health
>> J 5
>> D-68159 Mannheim, Germany
>>
>>
>
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