Le 08/05/2012 20:42, MCLAREN, Donald a écrit :
> You stated that you have a contrast between two conditions. This
> implied that you have one measurement per subject. Thus, the
> two-sample t-test would be appropriate.
>
> If you mean that you have two conditions that you want to compare
> between groups, then you have 2 measures per subject. Thus, the
> flexible factorial would be appropriate. Remember that you will only
> be able to compare the two conditions or the group*condition
> interaction. The effect of group is invalid.
>
Dear SPMers,
We are facing a similar problem as we have to analyse an O15-H20 PET
study with the following specifications:
2 groups: ON medication / OFF medication
3 conditions: Task1 / Task2 / Control (2 or 3 repetition by task)
We defined our design matrix with a flexible factorial design as follows
Factor 1: subjects
Factor 2: Group
Factor 3: Tasks
Our main questions here, are to compare:
Task 1 vs Control between ON & OFF groups
Task 2 vs Control between ON & OFF groups
there are different options concerning the effects to include in the Matrix
Main effect of subject (Factor 1)
Main effect of Group ?
Main effect of Tasks ?
interaction Group x Task (Factor 2 x Factor 3)
In the case of interaction Group x Task, the contrast weights we need to
define are different whereas the main effects are modelized or not...
More specifically, if Main Effects are included, it's impossible to test
only the interaction term: the contrast manager seems to wait for
weights in the Main effects columns too.
Any suggestions to help us analyze these data are welcome...
Thanks for your help
J.R
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Jérôme Redouté
Ph.D. - Ingénieur de Recherche - Université Claude Bernard - Lyon1
CERMEP - Imagerie du vivant
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