[FieldTrip] one/two-tails analysis

Sysoeva, Olga Vladimirovna sysoevao at psychiatry.wustl.edu
Mon Feb 13 21:49:17 CET 2012

Dear group,

I do face some weird results during my analysis.
Usually I do t-test analysis to compare 2 condions/groups and use 2-tails, as far as I do not have any preliminary hypothesis about the direction of the effect.
So my script looks something like Script 1 (see below). Important here is that I do keep the default parameters, cfg.tail = 0; cfg.clustertail = 0;
To my understanding of statistic, if you have the two-tail effect, and group 1 has bigger mean than gr2, you should have the effect with one-tail as well and even more significant. But what I discovered just today that if I change this parameter to 1, cfg.tail = 1; cfg.clustertail = 1; in order to look spesifically for the one-tail effect (gr1 is bigger than gr2), my very significant 2 tail effect just disappeared completely. Is it a bug?

Your comments are appreciated.
Best reGards,

Script 1
cfg = [];
cfg.method = 'montecarlo';       % use the Monte Carlo Method to calculate the significance probability
cfg.statistic = 'indepsamplesT'; % use the independent samples T-statistic as a measure to evaluate
                                 % the effect at the sample level
% cfg.statistic = 'indepsamplesF'; % use the independent samples T-statistic as a measure to evaluate
% cfg.computecritval = 'yes'                                 % the effect at the sample level
cfg.correctm = 'cluster';
cfg.clusteralpha = 0.05;         % alpha level of the sample-specific test statistic that will be used for thresholding
cfg.clusterstatistic = 'maxsum'; % test statistic that will be evaluated under the permutation distribution.
cfg.minnbchan = 1;               % minimum number of neighborhood channels that is required for a selected
                                 % sample to be included in the clustering algorithm (default=0).
cfg.tail = 0;                    % -1, 1 or 0 (default = 0); one-sided or two-sided test
cfg.clustertail = 0;
cfg.alpha = 0.05;               % alpha level of the permutation test
cfg.numrandomization = 1000;      % number of draws from the permutation distribution
cfg.neighbourdist = 0.2;
%cfg.neighbours      =  cfg_n;
cfg.channel          =  {'all', '-HEOG', '-VEOG', '-EKG'};
cfg.latency          = [0. 0.5];
cfg.layout           = 'C:\Matlab_programs\Fieldtrip\My_Fieldtrip\EEG1010_modified32.lay';

subj1 = length(GROUP {nGroup1,1});
subj2 = length(GROUP {nGroup2,1});
design = zeros(1,subj1+subj2);
design(1,1:subj1)        = 1;
design(1,subj1+1:subj1+subj2) = 2;
cfg.design   = design;
cfg.ivar     = 1;
[stat] = ft_timelockstatistics(cfg, peak1_gr1 , peak1_gr2 );

Olga Sysoeva,
Research Associate, PhD
Psychiatry Department,
Washington University School of Medicine
Campus Box 8134
660 South Euclid Ave
Saint Louis, MO 63110-9909

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