I understand that your contrast is a t-contrast. T-constrast have a
direction, for this reason it is used when we compare between groups,
while F-Contrast is for showing an effect of interest.
If you have group 1 .vs. group 2 and put a t-constrast 1 -1 you are
supposing that group 1 is major that group 2. If, in the contrary you
créate a t-contrast -1 1 you are supposing that the effect of the second
group is major. In other words, if you have significances in the
contrast 1 -1, the meanign is that you have more activity in the group 1
than group 2. In the other case, the group 1 shows less activity that
the group 2. These meaning is more or less, it depends that the study
that you are doing.
I hope that I help you!
If I am wrong, please SPM users, tell me!
INCLIVA Public Health Institute
El 2017-05-22 11:42, Sascha Frölich escribió:
> Dear all,
> I have a question regarding the two-sample t-test in SPM:
> When comparing group 1 vs group 2 (ie. contras 1 -1) I get a different
> result from when I use contrast -1 1 (group2 vs group 1, as far as I
> understood). How is this possible? Shouldn't the t-test by symmetric?
> What do the different results mean?