Jeanette,
I've compared the average of cope1 (A-B) with the negative contrast of cope2 (B-A) for completeness, and it's still the same problem...
Best,
Stephane
Stéphane Jacobs - Chercheur post-doctorant / Post-doctoral researcher ImpAct - Inserm U1028 - Equipe Pélisson Centre de Recherche en Neurosciences de Lyon 16 avenue du Doyen Lépine 69676 Bron Cedex, France Téléphone / Phone: (+33) (0)4-72-91-34-20Le 12.06.13 17:24, Jeanette Mumford a écrit :
Hi,
To be sure I understand your question, you are saying that at the second level, when you are running a within-subject analysis to combine data over multiple sessions for that subject, the average of A-B is not the same as the negative of the average of the B-A contrasts? To be clear, say cope1 is A-B and cope2 is B-A and the second level model, assuming 3 runs, is111
with the [1] and [-1] contrasts. You have data where the positive contrast for cope 1 (A-B) at the second level is not equal to the negative contrast for cope 2(B-A) at the second level? This doesn't seem mathematically possible. Not that it should matter, but I assume you're using fixed effects at the second level?
Cheers,
Jeanette
On Wed, Jun 12, 2013 at 4:46 AM, Stephane Jacobs <[log in to unmask]> wrote:
Hello,
I have what seems a trivial question but could not find an answer in the archives, so here it is...
I have several runs per subject, each containing all of my conditions. I set up all the contrasts of interest at the first level, and then just do a cross-session average at the second level for each subject. In the contrasts I chose, I usually have a contrast and it inverse: A-B and B-A. As expected, at the first level tese give me inverse statistical maps. However, when I run the cross session average at the second level, this is not true any more, some supposedly inverse contrasts even giving sometimes very similar maps...
Is there a reason why this should happen? Would it be better to approch this by averaging only the A-B contrasts at the second level, and defining the B-A contrast as its inverse at the second level (rather than average the first level B-A)?
Thanks for any help or advice!
Stephane