unfortunately, I am sort of naive, from a statistical point of view. Some simple questions:
The experiment is the following: three different groups performed a standard encoding task. I am interested in activation differences between these groups. For that, I used a ANOVA design, 1 factor with 3 levels (group 1, group 2, group 3).
QUESTION 1: Is that appropriate?
To look at general group differences, I used the following F-contrast
1 -1 0
1 0 -1
0 1 -1
This gave one nice cluster. In my opinion, this means that at least two groups have activation differences in this cluster.
QUESTION 2: Is that the correct interpretation?
Now I wanted to find out which groups differ. I used, as post-hoc t-tests, simple t-contrasts and found out that group 1 has more activity than group 3 in the nice cluster from above. However, these t-contrasts showed also other activation differences between the groups. I assume however that I am not allowed to take these differences as "statistical real" since they did not appear in the ANOVA F-contrast.
QUESTION 3: Is that correct?
At last, I still did not know how to interpret these group differences. It might be that
- both groups have activation, one more than the other.
- both groups have deactivation, one more than the other.
- one group has activation, the other deactivation.
This is probably not unimportant to know.
I tried several options:
1.) I used the "plot button", fitted response, against scan/time. This showed me nicely that group 1 differs from group 3, but I had a bad feeling: I did not know whether to use the "adjusted" or "fitted" response, I did not know how to extract the data from the graph, I did not know what SPN does to the data, etc.
2.) I used the "eigenvariate button" to extract the first eigenvariate from all voxels within my cluster. This gave totally different results. Does it only work when I want to look at a real time series?
3.) I decided to go back to the raw data, extracted the mean intensity value in the cluster from the con-images of all subjects. Again, I got different results
QUESTION 4: what is the right way to proceed?
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