[FieldTrip] Evaluation under the permutation distribution positive/negative for two-tailed analysis

[Oliver Hermann]

Hi all,

I wonder if anyone can clear up some confusion regarding evaluation of summed-cluster statistics following permutation.

Specifically, as of Maris and Oostenveld (2007), evaluation is performed for all clusters, based on the permutation distribution of the max (absolute) cluster statistic, see "It is important to note that the p-values for all eight clusters are calculated under the permutation distribution of the maximum (absolute value) cluster-level statistic and not under the permutation distribution of the second largest, third largest, etc."

However, when I look at the guide at https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.fieldtriptoolbox.org%2Ftutorial%2Fstats%2Fcluster_permutation_timelock%2F&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cc286b1b639a44637f3cb08dda9baac6a%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C638853341443615699%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Wl3JDW0ByghfeGsgL2yjb14frz3V0%2BLvert7tURrZsg%3D&reserved=0

the following recommendation is given:  And third, the critical value for the cluster-level test statistic (determined by cfg.alpha; see further) is now two-sided: negative cluster-level statistics must be compared with the negative critical value, and positive cluster-level statistics must be compared with the positive critical value.

To me, these two statements seem contradictory. Please could someone clarify the correct approach (I assume the latter) and why the discrepancy?

Thanks,
Oliver






-------------------

Oliver Hermann

PhD Researcher

Department of Psychology

University of Bath


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