[FieldTrip] Number of randomizations + 1 in clustering tests?

["Jörn M. Horschig"]

Hi Mariana,

yes, this is the case. You can think of it as your original data also 
being an observation, thus needs to be added to the number of 
permutations. If I remember correctly, this is for example explained 
here in more detail (and probably in more correct terms): 
http://www.statsci.org/smyth/pubs/permp.pdf

Best,
Jörn

On 3/5/2014 2:37 PM, Mariana Rebelo wrote:
> Dear all
>
> I am computing some clustering tests on my MEG data and I don't 
> understand the p-value returned in the field stat.posclusters(1).prob
> If I perform 1000 randomizations I get a p-value of 0.00799201
> I don't understand how this is possible, the p-value should be equal 
> to integer/1000, so I should get either 0.0070 or 0.0080
> It seems that in fact I am not performing 1000 randomizations as I set 
> in numrandomization, but 1001, because then: 8/1001 = 0.00799201
>
> Is this really the case? Is the fieldtrip script adding another 
> randomization to what is set by the user?
>
> Thanks!
> Mariana
>
>
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-- 
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
FieldTrip Development Team

P.O. Box 9101
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