<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Stephan,<div><br></div><div>to build an appropriate null distribution, the resampling procedure must stick to the structure of your experimental design. In the case of a group x condition (repeated measure) interaction, the groups are independent but the conditions are not. To take that constraint into account, I think you need to resample from each group independently. However, for each group, because of the paired nature of the design, when you sample 1 subject, you have to sample both conditions from that subject (and all the time points and sensors), i.e. you sample subjects with replacement independently from each group, but keeping paired data points together. </div><div>The strategy described here relies on a sampling with replacement under H0. In that case H0 is created by centering the data independently in each of the 4 cells of your design. So you start with a true null hypothesis and through resampling you can derive a table of F values for the interaction under H0.</div><div><br></div><div>Hope this help,</div><div><br></div><div>GAR</div><div><br></div><div><br></div><div><br><div><div>On 24 Nov 2009, at 09:19, Stephan Moratti wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Hi all,<br><br>Thanks for the interesing discussion about the interaction with respect to <br>permutation. I have been also struggling with this question. As we want to <br>test the interaction (and not the main effect condition), I am not sure if <br>resampling condition would produce the distribution of our null hypothesis. <br>Regarding the interaction, we want to check if the mean values across <br>conditions follow the same pattern in two groups (if we consider a group x <br>condition interaction). If we make a line plot an interaction would be indicated <br>by a line crossing if each line represents a group (ideally). No interaction <br>would be represented by parallel lines for each group. So I wonder if by <br>resampling the subject values between the groups keeping the condition <br>structure intact, would create our null distribution for the interaction. If we <br>resample condition, we would destroy the condition structure.<br><br>What do you think? I would be happy for any input.<br><br>Best,<br><br>Stephan<br><br>----------------------------------<br>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. See also <a href="http://listserv.surfnet.nl/archives/fieldtrip.html">http://listserv.surfnet.nl/archives/fieldtrip.html</a> and <a href="http://www.ru.nl/neuroimaging/fieldtrip">http://www.ru.nl/neuroimaging/fieldtrip</a>.<br></div></blockquote></div><br><div apple-content-edited="true"> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 18px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; min-height: 14px; "><br class="Apple-interchange-newline"><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">************************************************************************************</div><b style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: normal; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><b style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; "><span class="Apple-style-span" style="font-weight: bold; ">Guillaume A. 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<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>