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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Hi Jan,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>If you get confused about the literature then I am partly to
blame. <o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Things are different for single subject studies (units-of–observation
are trials) and multiple subject studies (units-of-observation are subjects).
You describe a multiple subject study of the within-subjects type (every
subjects participates in both experimental conditions). For a single channel
pair and a single frequency bin, the appropriate statistic is the dependent
(paired) samples t-statistic or, in a nonparametric framework, the Wilcoxon signed
rank sum test. (You mention the Wilcoxon rank sum test, but this is for a study
of the between-subjects type.) You apply this test statistic to the
condition-specific coherences, obtained by summing and normalizing the trial(taper)-specific
cross-spectra. This statistical test works fine (asymptotically, for the
dependent samples t-statistic) if the number of trials in the two conditions
are equal. The requirement of an equal number of trials follows from the fact
that coherence is a biased measure, with the bias being dependent on the number
of trials.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>The situation is different for a single subject study. You may
want to have a look at our paper (Maris, Schoffelen, Fries, 2007), which also
deals with the multiple comparisons problem.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Best,<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Eric<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>dr. Eric Maris<br>
Donders Institute for Brain, Cognition and Behavior<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Center for Cognition and F.C. Donders Center for Cognitive
Neuroimaging<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Radboud University</span><span lang=EN-US style='font-size:11.0pt;
font-family:"Verdana","sans-serif";color:#1F497D'><br>
</span><span lang=EN-GB style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>P.O. Box 9104</span><span lang=EN-US style='font-size:11.0pt;
font-family:"Verdana","sans-serif";color:#1F497D'><br>
</span><span lang=EN-GB style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>6500 HE Nijmegen<br>
The Netherlands<br>
T:+31 24 3612651<br>
Mobile: 06 39584581<o:p></o:p></span></p>
<p class=MsoNormal><span lang=DE style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>F:+31 24 3616066<br>
E: e.</span><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><a href="mailto:e.maris@donders.ru.nl"
title="blocked::mailto:maris@nici.kun.nl"><span lang=DE>maris@donders.ru.nl</span></a></span><span
lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> FieldTrip discussion list
[mailto:FIELDTRIP@NIC.SURFNET.NL] <b>On Behalf Of </b>Jan Hirschmann<br>
<b>Sent:</b> donderdag 5 augustus 2010 15:50<br>
<b>To:</b> FIELDTRIP@NIC.SURFNET.NL<br>
<b>Subject:</b> [FIELDTRIP] significance of coherence differences<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span lang=DE style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Dear
fieldtrip experts,<o:p></o:p></span></p>
<p class=MsoNormal><span lang=DE style='font-size:10.0pt;font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:10.0pt;font-family:"Arial","sans-serif"'>I
am getting more and more confused about the literature of significance tests of
coherence differences. The one essential point I did not quite understand is
why conventional tests do not seem to apply. Lets say I have two vectors of
mean alpha band coherences, relating to two experimental conditions (continuous
data, no trials). The vector entries stem from different subjects:
C1=[meancoh_cond1_subj1, meancoh_cond1_subj2,…] and C2=[meancoh_cond2_subj1,
meancoh_cond2_subj2,…]. What keeps me from comparing the two vectors,
say, with a Wilcoxon ranksum test?<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:10.0pt;font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Thanks
for any comments,<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:10.0pt;font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-GB style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Jan
Hirschmann<o:p></o:p></span></p>
<p>----------------------------------<o:p></o:p></p>
<p>The aim of this list is to facilitate the discussion between users of the
FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and
EEG analysis.<o:p></o:p></p>
<p>http://listserv.surfnet.nl/archives/fieldtrip.html<o:p></o:p></p>
<p>http://www.ru.nl/fcdonders/fieldtrip/<o:p></o:p></p>
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<p>----------------------------------</p>
<p>The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis.</p>
<p> http://listserv.surfnet.nl/archives/fieldtrip.html</p>
<p> http://www.ru.nl/fcdonders/fieldtrip/</p>