[FieldTrip] One-sided versus two-sided cluster statistics
Nina Kahlbrock
Nina.Kahlbrock at uni-duesseldorf.de
Mon Feb 21 18:54:27 CET 2011
Dear all,
I have a question concerning cluster based permutation statistics.
It is more a basic question on the difference between one- and two-sided
testings. I recall from simple t-tests that as an approach to your two-sided
p-value you can multiply your one-sided p-value by 2. I hope this is
correct? And I am assuming this to also hold in the other direction, thus,
approximating your one-sided p-value by dividing your two-sided p-value by
2?
That is why I am expecting something similar when calculating my statistics
with a cluster based permutation approach.
So what I am actually doing is comparing two conditions at one sensor,
comparing time-frequency data.
When I calculate a two-sided dependent samples t-test
(cfg = [];
cfg.channel = 'all';
cfg.latency = [0.5 1.5];
cfg.avgoverchan = 'no';
cfg.avgovertime = 'no';
cfg.parameter = 'powspctrm';
cfg.method = 'montecarlo';
cfg.statistic = 'depsamplesT';
cfg.correctm = 'cluster';
cfg.clusteralpha = 0.05;
cfg.clusterstatistic = 'maxsum';
cfg.tail = 0;
cfg.clustertail = 0;
cfg.alpha = 0.025;
cfg.numrandomization = 1000;
)
, I find one positive cluster with a p-value of 0.036, thus not significant
(see fig 1).
When I then change my settings to a one-sided test
(cfg.tail = 1;
cfg.clustertail = 1;
cfg.alpha = 0.05;)
, the positive cluster gets a p-value of 0.056, again not significant (see
fig 2).
With the assumption described above, however, I would expect this to become
significant when using a one-sided test. Is my assumption correct? If not,
could anybody comment on what is wrong about my assumption?
I would be very grateful for any advice!
Kind regards and thank you very much in advance!
Nina
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Nina Kahlbrock
Institute of Clinical Neuroscience and Medical Psychology
Heinrich Heine University Duesseldorf
Universitaetsstr. 1
40225 DÃ¼sseldorf
Tel.: +49 211 81 18075
Fax. .: +49 211 81 19916
Mail: <mailto:Nina.Kahlbrock at med.uni-duesseldorf.de>
Nina.Kahlbrock at med.uni-duesseldorf.de
<http://www.uniklinik-duesseldorf.de/medpsychologie>
http://www.uniklinik-duesseldorf.de/medpsychologie
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