[FieldTrip] One-sided versus two-sided cluster statistics

[Nina Kahlbrock]

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

 

 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - -

 

Nina Kahlbrock

Institute of Clinical Neuroscience and Medical Psychology 

Heinrich Heine University Duesseldorf

Universitaetsstr.  1

40225  Düsseldorf

 

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

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110221/f320bb6e/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fig 1.pdf
Type: application/pdf
Size: 9504 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110221/f320bb6e/attachment-0002.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fig 2.pdf
Type: application/pdf
Size: 9660 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110221/f320bb6e/attachment-0003.pdf>