Dear FSL users and experts,
Sorry for posting a similar question in a short interval, but I am running out of time and couldn't find a solution yet.
We compared the whole-brain GM between 3 groups of subjects in FSL-VBM, in an ANOVA design, with age and gender as covariates. TFCE was used to display results. While we had some positive results, no differences were found between 2 of these 3 groups.
We reported our findings and we were asked 'to test formally if the negative results were due to statistitcal power' by a reviewer.
Thanks to previous posts in this email list and inputs in response to my previous question, I am aware that post-hoc power analyses may be meaningless, however my plan is to include a power analysis (because we were asked to) and point out this controversy in the response, which I think is a safer way to deal with this request.
So my questions start here:
1. To calculate the power I know that I need the effect size (cohen's d) and sample sizes for a regular t-test. Is that the same for FSL-VBM analysis ?
2. If so, what is the best way to get the effect size for a VBM analysis ?
-I can think of two ways to do this: by using fsl_glm i) to get PEs or ii) t-scores and calculate effect size by using t-scores with sample sizes. Do you think these are correct ?
-I tried: fsl_glm -i GM_mod_merg_s3 -d design.mat -c design.con -m GM_mask -o PE --out_t=t_scores
which I think gave me a single PE image and t-score image for the model?
--But my design includes 6 contrasts and I am interested in the effect size for only 2 of those contrasts. How can I get the PEs and the t-scores for these two contrasts separately ?
--And what is the relation of the scores on the PE image with cohen's d ?
I would very much appreciate your inputs. Please try to be basic while guiding me since I am a novice both in statistics and FSL.
Many thanks,
Sinan
|