Hi all,
Just to jump on Mike's comment on wether an initial p=0.005 (+ cluster-wise correction) would be ok or not?
I understand there are less risks to report false positive blobs using p=0.001 as an initial threshold, and the need to (at least) use a strict p=0.001 (+ cluster-wise correction) when looking at within group activation and/or correlation. However, would it become a problem to use an initial threshold of p=0.005 (+ cluster-wise correction) when looking at, say for instance, 2x2 (between groups) ANCOVAs with clinical populations? This more liberal threshold will impact spatial sensitivity, but will it impact the validity of the findings?
Thanks.
Yann
On 14 Jul 2016, at 6:31 pm, Mike <[log in to unmask]> wrote:
> Thanks for everyone's replies. However, I believe that many researchers who use fMRI analysis software are not with a firm statistical background, just like me. For practical reasons, we need a "guideline," if any, to control multiple comparisons problem. Concerning cluster-wise thresholding, below is what I would follow according to Woo et al., 2004 and the recent cluster failure paper in PNAS, but I hope some erperts here can comment a bit.
>
> (1). For SPM and AFNI 3dClustSim users, the first arbitrary cluster-forming threshold (CFT) is suggested to be not too lenient. 0.001 is good, but 0.01 is definitely poor (I have no idea if 0.005 is ok or not?). Then you can report clusters that survive a FWE-corrected p<.05 at the cluster-wise level (but can I report FDR-corrected p<0.05?). The commonly used "P = 0.001 uncorrected with a k of 10 voxels" should be abandoned.
>
> (2). The commonly used "P = 0.001 uncorrected with a k of 10 voxels" should be abandoned (but it seems that many people still use it...).
>
> Besides, I have a naive question: since cluster-extent based thresholding might be more problematic, why don't we just stick on voxel-wise thresholding?
>
> Mike
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