[Eeglablist] Bootstrap significance probability level

Christopher Summerfield summerfd at paradox.psych.columbia.edu
Wed Nov 2 05:29:41 PST 2005


dear Adrian, the familywise error rate can be calculated for bootstrap
stats in the following way:

1) calculate your statistics at every timepoint/voxel/whatever (we'll call
these 'point estimate statistics')
2) shuffle and permute
3) for each permutation, enter the *maximum* permuted value from across
all the sampled timepoints/voxels into a distibution
4) 'point estimate' statistics falling within the 95% confidence interval
of this distribution can be considered significant, adjusted for
familywise error.

for more information, see Burgess & Gruzelier, 1999
Methodological advances in the analysis of event-related desynchronization
data.	in Event-Related Desynchronization ed Gert Pfurtscheller, F H
Lopes Da Silva.

this process could be adapted to provdide false discovery rate correction
(see Genovese et al 2002).
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11906227&query_hl=1

good luck,
Chris



Christopher Summerfield
summerfd at psych.columbia.edu

On Mon, 31 Oct 2005, Adrian Guggisberg wrote:

> Hello
>
> I wonder, whether anyone has some practical considerations about which level
> of significance probability should be chosen for bootstrap statistics.
>
> Since we have to deal with multiple comparisons, the significance threshold
> should be decreased, but the question is how much. As much as I understand,
> the Bonferroni correction for multiple testing would correspond to a level
> of p < 0.05/(number of time points * number of frequency bins). However,
> this would require a very high amount of surrogate data.
>
> Most of the studies seem to use p-values < 0.01. What's the argumentation
> for this?
>
> In their J Neurosci Meth; 2004:9-21 Paper, Arnaud Delorme and Scott Makeig
> write that they have
> "implemented a method to fit the observed data distribution using a forth
> order distribution fit. This feature will be available in a near-term release
> of EEGLAB."
> Does this feature allow a significance level of 0.05? Is there a way to obtain
> it?
>
> Thank you very much for your help.
>
>
> Adrian Guggisberg, MD
> Neurologische Klinik und Poliklinik
> Inselspital
> CH-3010 Bern
> Switzerland
>
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