In a 2x2 design, and you are are looking at the main effects; you only need to have local sphericity for each factor. Based on simulations, I would not assume that local sphericity exists. The safest approach is to use SPM where you can correct for sphericity violations or eliminate 1 factor by subtracting the two images (or averaging the two images) depending on what you want to test.
Keep in mind that in this design you can only test the difference in levels and not average of both levels against 0.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Tue, Feb 14, 2012 at 4:23 AM, Stéphane Jacobs
<[log in to unmask]> wrote:
Sorry for multiple posting, my previous response did not seem to appear correctly...
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Dear Donald,
Many thanks for your response, this is clearer now.
Just to make sure I understand this for future analyses, is the reason why the FEAT manual gives teh 2x2 repeated measures ANOVA example because the sphericity assumption is always met for 2 level factors?
Thanks again,
Stephane
