Dear Reader,
We have collected data on subjects receiving sips of chocolate milk [A1] and sips of water [A2] over 3 sessions [ B1 B2 B3].
First I made a [A1 - A2] contrast per session, than on second level I analysed the main effect in a 1x3 full factorial model.
However, as the contrast was already made on first level, the second level was a 1x3 model rather than 2x3 and no interaction effects could be calculated, i.e. difference between drinking chocolate milk and water over the 3 sessions.
Than I considered making an interaction model on first level, because of the problem described above, and because I understood this is better concerning the degrees of freedom. I am not sure how to do this correctly as I have 3 sessions.
I found this explanation example 2 - 3 factors: https://en.wikibooks.org/wiki/SPM/Group_Analysis
there they describe to:
make two contrasts on first level: ( I am not sure wheter this is a t-contrast or f-contrast, I am assuming it should be a t contrast)
A1B1 A1B2 A1B3 A2B1 A2B2 A2B3 contrast1 [ 1 -1 0 -1 1 0 ] and contrast2 [0 1 -1 0 -1 1].
Than they analyse this using a two-sample t test with F contrast [ 1 0 ; 0 1]
This should than give the interaction effect between stimuli [A1 and A2] and session [ B1, B2 B3] .
Do you think this is the right way of analyzing this?
Thank you in advance,
Marlou
|