Dear SPM users,
I would like to rise a question about the validity of group analyses in SPM when using within-subject designs with more than one within-subject factor. Although the topic has been discussed various times (see threads about "Flexible factorial", "x by x - ANOVA" and so on) , I somehow have the feeling that there is no definite answer so far. This is somewhat confusing especially for beginners as there are hundreds of fMRI papers by now.
When looking at published papers dealing with two and more within-subject factors, some explicitly or implicitly use Flexible factorial, others seem to use Full factorial, and others don't state which model they use. Some seem to add "subject" as a between-subject factor, other papers seem not. I have been talking to people who are into fMRI successfully for many years. They are no statistical experts, which means that they might run incorrect analyses in a pure statistical sense, but they probably represent the behavior of "normal" fMRI researchers.
In summary, there seem to be two "popular" options:
Option 1 is to use "Full factorial" setting "independence" to "no" and "variance" to "equal" (without "subject" as a factor)
Option 2 is to use "Flexible factorial" (in general with "subject" as a factor)
I well understand the different statistical meaning of these models. But nonetheless both options are published regularly (at least I got this impression when looking at various papers). Is this just due to lack of knowledge/disinterest of reviewers? In case one would want to publish his/her own analysis, which of the two options mentioned above (or any other) would you prefer? Could one justify his/her own "incorrect" analysis by refering to other "incorrect" studies (so keeping with a tradition ;-) ?
I'm aware of the scripts known as GLM flex (http://nmr.mgh.harvard.edu/harvardagingbrain/People/AaronSchultz/Blog/Blog.html), which work with different error terms and help to overcome the problems with traditional SPM analyses. Donald McLaren has written a lot about it in previous threads. Still, I somehow was a little surprised that there was no real discussion about the meaning for studies already published. I also wonder whether SPM12 is going to adopt the options of SPM8?
Any comment is greatly appreciated!
Gabor
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