Dear Mugundhu,
On Tue, 1 Dec 2020 13:33:16 +0530, Mugundhu RB <[log in to unmask]> wrote:
>Dear Experts,
>I am analyzing my structural MRI data. I have 3 groups, A (n=28), B (n=28,
>C (n=27) and 4 co-variates. I used the Full Factorial Design model in CAT12
>for statistical analysis. It has generated F contrast automatically. From
>the F test (main effect of ANOVA) result, I have selected 8 regions to
>create a single union mask. I used this mask for SVC in TFCE analysis.
This might be a typical case of double-dipping:
https://dx.doi.org/10.1038%2Fnn.2303
Small volume correction is thought to limit your statistics to those regions where you have a clear prior anatomical hypothesis. It is not appropriate to select the SVC area based on a statistical test in the same design.
>For group comparison, example;
>For A>B I used the contrast 1 -1 0 0 0 0 0
>For B>C I used the contrast 0 1 -1 0 0 0 0
>I kept TFCE permutation as 5000
>
>I have a few questions.
>For group comparison, the contrasts which I mentioned here are correct?
If your thirst three columns represent the three groups, yes. Please also check the CAT12 manual for defining contrasts.
>TFCE with 5000 permutations is sufficient for 3 group comparison or should
>I increase the number of permutation?
5000 permutations are a good starting point to get some first results. This is also mentioned in the help text (In order to obtain reliable estimates you need about 5000-10000 permutations.)
For your final analysis, a higher number of 20000 iterations might be better to be on the safe side.
>Should I do any additional statistics corrections?
If you finished the permutations there are also maps saved for FDR- and FWE-correction. Please check the online help of the TFCE toolbox for the naming convention of the result files.
Best,
Christian
>
>Please advise me.
>Thank you
>Mugundhu
>
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