Hi,
It seems like a number of people have a common question about how to set up
a 2x2 analysis with one repeated measures variable and one between subjects
variable (a pretty common design in neuroimaging). The example with the 1X4
was very helpful. Can someone please expand to 2x2?
Thanks for your help.
Jatin
>>> Dear Michiru,
>>>
>>> This is most easily done using the 'Flexible Factorial'
>>> option.
>>>
>>> 1. Create two factors.
>>>
>>> 2. Call the the first one Subject. Independence Yes, Variance Equal.
>>>
>>> 3. Call the second one 'Condition'. Independence Yes, Variance Unequal.
>>>
>>> 4. Under, Specify Subjects or all Scans, Choose Subjects
>>>
>>> 5. Under Subjects, create a new 'Subject' for each subject that you have
>>> eg. 5.
>>>
>>> 6. Then, for each Subject, under 'Scans'. Enter the 4 scans you have for
>>> each subject.
>>>
>>> 7. Also, for each Subject, under 'Conditions' enter the vector [1:4]
>>>
>>> 8. Under Main effects and Interactions create 2 main effects; factor 1
>>> and factor 2.
>>>
>>> 9. Specify other covariates as necessary and your o/p directory.
>>>
>>> 10. Then save your design job as 'within_subject_design' and press run.
>>>
>>> I have attached my saved job file 'within_subject_design.mat' as a
>>> template for you. When you run it, SPM should create the design matrix
>>> shown in 'design-matrix.png'.
>>>
>>> Note the 5 subject columns on the left. Without these 5 columns
>>> you do not have a 'within-subject' design.
>>>
>>> Also I have treated your 2 x 2 design as a 1 x 4. So you'll need to bear
>>> this in mind when doing your contrasts eg. 1 1 -1 -1 and 1 -1 1 -1 to
>>> test for main effects and 1 -1 -1 1 for the interaction (of course,
>>> pre-pad these with 5 0's).
>>>
>>> Best wishes,
>>>
>>> Will.
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