Dear SPM list
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 ?
Thank you very much for your answer
Emmanuel MELLET
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Dr E. MELLET MD PhD
Groupe d'Imagerie Neurofonctionnelle
UPRES-EA 2127 & CEA-LRC 13V
Centre CYCERON
BP 5229
14074 CAEN Cedex
FRANCE
Tel : 33 (0) 2 31 47 02 14
Fax : 33 (0) 2 31 47 02 22
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