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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Dear May,<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'><p class=MsoNormal style='margin-bottom:12.0pt'>2 subject groups: Controls (C) and Patients (P).<br>For a particular experimental condition (A), on average, subjects within each group have the following number of trials:<br>condA_Ntrials_C ~= 100<br>condA_Ntrials_P ~= 80<br><br>I plan to perform a 2-step statistics-test to test for differences between C and P groups on a particular experimental condition.<br>Using a 1st level within subject t-test (or active-vs-baseline test) on baseline vs active period for each group, deriving subject-specific t-stats which are to be subsequently used in a 2nd level test between subject group contrast, using nonparametric test.<br><br><span style='color:#3333FF'>Question 1) Does it matter if the average trials per subject in each group is different? Should I try to equalize the trial numbers by e.g. random removal of some trials before computing any statistics? Is this advisable?</span><br><br><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>Performing a second level t-test (in your case, between different groups of subjects) on first level t-tests is very unusual. In fact, this would imply that you second level t-test is about an hypothesis that pertains to first level t-tests. Instead, one always performs a second level t-test on first-level averages (typically baseline-normalized to deal with individual differences in distance to the helmet or baseline power). Differences in number of trials per conditions is never a problem as long as your first-level measure (the baseline-normalized mean) is unbiased.<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><br><span style='color:#3333FF'>Question 2) Minimum trials required for source-level significance?</span><br>The supplementary info. accompanying the 2007 paper illustrated some minimum trial numbers required for obtaining a significant effect at the sensor-level analysis, given some threshold (e.g. cluster>250 sensor-time pairs).<br>Has anyone systematically shown what is the minimum number of trials required to obtain significant clusters/fdr stats at the source-level analysis? <br>If not, is there a sensible way to find out? (presumably involving some form of bootstrapping and simulation?)<br><br><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>There is no sensible way to find out this minimum number of trials, because the true effect size is unknown.<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><br>This relates to another issue I have with regards to 'within-subjects' comparison for 2 experimental conditions A and B where there are average trial number differences both within and between subject groups: <br>condA_Ntrials_C ~= 100 condB_Ntrials_C ~= 60 (min. =35)<br>condA_Ntrials_P ~= 80 condB_Ntrials_P ~= 50 (min. =35)<br><br>A similar concern of uneven trials arises if say I wish to perform a within subjects comparison between the experimental conditions. <br><br><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>I think I’ve dealt with this question.<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><br>In general, Q: <span style='color:#3333FF'>Would the intended 2-step statistical test (as described above) be appropriate, OR would it best to control for 'equal' number of trials for all subjects and conditions of interest?</span><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>I think I’ve answered this.<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>Best,<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'>Eric Maris<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><br><br>I would really appreciate it if someone could kindly comment or offer advise where appropriate.<br><br>Thank you very much in advance for your time. <br><br>Yours sincerely,<br>May<br><br><o:p></o:p></p><div><p class=MsoNormal style='margin-bottom:0cm;margin-bottom:.0001pt;line-height:normal'>-- <br><br><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:0cm;margin-bottom:.0001pt'><span style='font-size:8.0pt;line-height:115%;color:#595959'>Heng-Ru <i>May</i> Tan</span><o:p></o:p></p><p class=MsoNormal style='margin-bottom:0cm;margin-bottom:.0001pt'><span style='font-size:8.0pt;line-height:115%;color:#595959'>Institute of Neuroscience and Psychology (INP) </span><span style='font-size:8.0pt;line-height:115%;color:#595959'>▫ Centre for Cognitive Neuroimaging (CCNi)</span><span style='font-size:8.0pt;line-height:115%;color:#595959'> </span><span style='font-size:8.0pt;line-height:115%;color:#595959'>▫ </span><span style='font-size:8.0pt;line-height:115%;color:#595959'>University of Glasgow</span><o:p></o:p></p><p class=MsoNormal style='margin-bottom:0cm;margin-bottom:.0001pt'><span style='font-size:8.0pt;line-height:115%;color:#595959'>58 Hillhead Street, Glasgow G12 8QB</span><span style='font-size:8.0pt;line-height:115%;color:#595959'> </span><span style='font-size:8.0pt;line-height:115%;color:#595959'>▫ +44 (0)141-330-5090</span><span style='font-size:8.0pt;line-height:115%;color:#595959'> </span><span style='font-size:8.0pt;line-height:115%;color:#595959'>▫ <a href="mailto:Heng-RuMay.Tan@glasgow.ac.uk">Heng-RuMay.Tan@glasgow.ac.uk</a></span><o:p></o:p></p></div></div></div></body></html>