Dear All,
After my paired t-test analysis, I would like to use Alphasim for multiple comparision correction....however, I'm not sure about the procedure...
From what I saw in the video, it seems to be 1) set the p value in slice viewer 2)check the list of alphasim correction provided in slice viewer and 3)enter in the cluster size value, etc accordingly....
1.so if I want to have p<0.05 (corrected) for my T stats image, I will need to first set the p value to 0.05 in slice viewer and then look up the cluster size in the list....but my concern is the "p on individual voxel"...what does this p value mean? is it corrected or uncorrected? will it affect the overall p value of my T stats image?
2.Also, does the p value in slice viewer need always be the same as the overall p value of the image? for example, if i want the T stats image to be p<0.01 corrected, then i should set the p value in slice viewer as 0.01 before i set up the cluster size etc?
I was also wondering about the rest alphasim function...
3.Supposedly my FWHM is 5mm when I perform smoothing.....so I will need to calculate a list with appropriate cluster size...
let's say if I want to have p<0.01 corrected for my t stats image, i will go to the rest alphasim and enter 5 for FWHM, 0.01 for p threshold and use the default mask and run.....and it should give me the output list?
I tested it out using p threshold 0.05, FWHM 4, rmm 4, mask:default mask, and assume the output list should give the same cluster size value as the one in slice viewer....however, I found they are not the same.
Thus I am confused on how to peform the alphasim correction for my t stats image....
Hope someone could help solve the problem...thanks in advance
Catherine
Submitted by jigongjun on Fri, 01/25/2013 - 17:31 Permalink
Re: Alphasim question
Hope these could help you.
1. "first set the p value to 0.05 in slice viewer". -------------This is the P value for individual voxel.
The overall P value is dependent on the combination of individual P value and cluster size value.
2. The p value in slice viewer does not need always be the same as the overall p value of the image.
For example, if the P value for individual voxel is 0.01, the overall value could be 0.05 by setting the cluster size value.
3. The P value in the AlphSim GUI does not mean the corrected overall P value. It is the P value for individual voxel.
The overall P value (e.g. 0.05) could be accomplised by individual p (e.g. 0.05) + cluster size value (e.g. 54) + rmm=4, FWHM=4, default mask, or
by individual p (e.g. 0.01) + cluster size value (e.g. 16) + rmm=4, FWHM=4, default mask.
The cluster size value is from the result of AlphSim program.