Since you have specified "All Scans", you need a factor matrix with 4 columns. There is no way for SPM to know which scans go with which subject if you leave it out. If you choose the other option, then you specify the other levels on a subject by subject basis and thus there is explicit knowledge of which scans go with which subjects.
Column 1: should list the observation/row number
Column 2: factor 1 level
Column 3: factor 2 level (if no factor 2, use 1s)
Column 4: factor 3 level (if no factor 3, use 1s)
e.g.
1 1 1 1
2 1 1 2
3 1 1 3
4 2 1 1
5 2 1 2
...
16 6 2 1
17 6 2 2
18 6 2 3
...
...
27 9 3 3
Hope this helps.
Hello.
I am following Glascher and Gitelman's guide for flexible factorial design and have run into a snag.
I am trying to set up a 3x3 factorial design where I have three groups and three time points. I want to include the main effects of subject, group, time and groupxtime interaction.
So I set up factors for subject, group, time and choose to specify all scans and factor matrix.
I have input the subjects list in order of group then time. Therefore my factor matrix should be (since the subject factor does not have to be specified as it is a special factor):
Group 1:
1 1
1 2
1 3
1 1
1 2
1 3
Etc.
Group 2:
2 1
2 2
2 3
2 1
2 2
2 3
Etc.
Group 3:
3 1
3 2
Etc.
Then I set three main effects for subject, group, and time, and the interaction of group x time.
This seems to be the right way to set things up but when I go to run this and create the design matrix I get the following:
Error: factor matrix must have four columns.
I am not sure why I get this error as I should not need four columns (I only have three factors overall and the subject factor should not need to be set). Can anybody tell me where I might be going wrong?
Thanks in advance for the help!
-John West
IU School of Medicine