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)
I always include the columns for the main effect because I was told that you should always include the main effect if you include the interaction term. From a practical standpoint, it doesn't seem to make much difference - although this might not generalize to all studies. The differences between including them versus not including them relate to the computation of the variance-covariance correction.
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.
The contrast for the interaction will be the same in both models, with the exception that when you have the main effect for group and task, you will need more 0s in the contrast. The non-zero elements over the interaction term will be 1 0 -1 -1 0 1 and 0 1 -1 0 -1 1 for your two questions. All other columns will be 0.
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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