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):
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!
IU School of Medicine