Dear all,

 

I have a question about testing average effects of condition in full factorial models. Let’s say I have three groups in a full factorial model. An F contrast called “average effect of condition” will be automatically created by SPM with these weights: [1 1 1]; it displays any effect (positive or negative) in the three groups.

If I build a F contrast in this way: [1 0 0; 0 1 0; 0 0 1], it seems to me that the average effects of condition are also displayed, because the pattern of activity is quite similar to the previous one, although not identical (my impression is that this latter contrast is more noisy).

Which is the conceptual difference between these two contrasts?

Thank you in advance!

Kind regards

Paola