[FieldTrip] ft_freqstatistics and FDR-correction

Patricia Wollstadt Patricia.Wollstadt at gmx.de
Mon Jan 24 17:47:26 CET 2011

Dear Fieldtrip-users,

I'm currently trying to calculate freqstatistics for four experimental groups, using the following options:

groupSize=[length(subjects{1}) length(subjects{2}) length(subjects{3}) length(subjects{4})];

design = [1:groupSize(1)+groupSize(2)+groupSize(3)+groupSize(4)];            % 1 to N subjects
design(2,:) = [ones(1,groupSize(1)) 2*ones(1,groupSize(2)) 3*ones(1,groupSize(3)) 4*ones(1,groupSize(4))]; % indep var -> group membership

cfg.uvar = 1;       
cfg.ivar = 2;    
cfg.method = 'montecarlo';
cfg.statistic = 'indepsamplesF';
cfg.correctm = 'fdr';
cfg.numrandomization = 180000;
cfg.alpha = 0.01/7;
cfg.tail = 1;
cfg.design = design;
cfg.channels = myChannels;
cfg.avgovertime = 'yes';
cfg.avgoverfreq = 'yes';
cfg.frequency = [1 3];

stat = ft_freqstatistics(cfg, group1avg, group2avg, group3avg, group4avg);

With these settings, the coputation itself runs fine, but the resulting  'prob'-matrix contains only zeros (accordingly the 'mask'-matrix consists of only ones). This would mean, that I found significant effects for all channels. Is this possible or rather due to an error somewhere in my script? Is it likely, that these are 'real' zeroes or rather very small values, close to zero?
Any hints and suggestions are very welcome, thank you in advance,

Kind regards

Patricia Wollstadt

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