 |
 |
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 ***************************** 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 ****************************** %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%