Dear SPM experts,

 

In case it might help finding where the issue might come from, I noticed that I encounter the same sort of results (i.e. exactly similar p-FDR values) also when using topological correction (and looking at the peak level, but also for two clusters in my first contrast when looking at the cluster level), and that this happens both in SPM 8 and SPM 12.

 

Again, this seems to point to the spm_p_FDR.m way of selecting p-values.

 

Any input would be greatly appreciated.

 

Kind Regards,

 

 

Dear SPM experts,

 

I’ve encountered strange results concerning FDR correction.

To start with, let me clarify that I refer to the voxel-wise (i.e. non-topological, with stats.topoFDR set to 0) FDR correction, and that I am only looking at the “peak” p-columns in SPM.

So, when using an uncorrected threshold of p<.001 with k=10, resulting peak FDR values are very similar.

 

In one contrast, of my 18 resulting peak, 17 FDR p-values are exactly 0.026131713.

From what I have understood, FDR-correction is calculated from uncorrected p-values. However, the p(unc) leading to the same FDR results range from 0.00000398 and 0.00000744.

Although they are very close, they are not exactly similar.

 

In another contrast, of my 15 resulting peak, five are exactly 0.118333366538248, three exactly 0.138802207216991, two exactly 0.135174599683222 and two exactly 0.152021262602001. They are not in the same cluster.

Again, p-values are very close but not exactly the same.

 

I think it has to do with the way spm_P_FDR defines the FDR p-value (min(QS(I:S))

I tried calculating the Qs by hand starting from the uncorrected p-values, but I could not.

 

All in all, I do not understand why I get those strange results and could not find similar thread in mailing list.

Would anyone have any idea of why those FDR p­-values are that similar, and whether they are valid?

 

Kind Regards,

 

--
Maxime Résibois

PhD Student (KU Leuven)
Quantitative Psychology and Individual Differences
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tel. +32 16 37 30 98