Dear Emmanuel,
> I'm trying to compute a conjunction analysis using SPM99b including
> three studies.
>
> When I use an UNCORRECTED p value of 0.001, the corresponding height
> threshold is T = 3.23. In the "Statitistics: volume summary table" it
> is stated that both uncorrected AND corrected p values are thresholded
> to p < 0.000. In this case, no p value (corrected and uncorrected)
> exceed 0.000: very few voxels survived to this analysis.
>
> When I use a CORRECTED p value of 0.05, the corresponding height
> threshold is T = 2.36. In the Statitistics: volume summary table it is
> stated that the corrected p value is 0.05. As a matter of fact, in this
> case the lower corrected p value (as given is the volume summary) is
> 0.017 and much more voxel are listed than in the former uncorrected
> threshold.
>
> Thus, it seems that in the conjunction analysis the use of a corrected
> threshold is more permissive than the use of an uncorrected
> threshold. Note that in a single contrast analysis the p<0.05
> CORRECTED threshold leads to a T value of 4.95 while the p<0.001
> UNCORRECTED value leads to a T value of 3.23. In this latter case, the
> use of a corrected p value is obviously more stringent.
>
> How do you explain this discrepancy ?
This is a recurring point of confusion (that has been addressed in
previous emails) and may prompt us to change the user specification.
Briefly:
1) If you specify an uncorrected threshold (say 0.001) then this
threshold is applied to all (say n) contrasts entering into the
conjunction. The resulting uncorrected p value for any voxel surviving
the conjunction is therefore less than 0.001 to the power of n. To get
a conjunction p value (uncorrected) of 0.001 then enter 0.001^(1/n)
(e.g. 0.0316 for n = 2) as the original threshold.
2) If you specify a corrected threshold (say 0.05) then this is the
conjunction p value and the threshold applied to the individual
contrasts is computed for you, using Gaussian field theory.
I hope this clarifies things and I am sorry for the confusion - Karl
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