<div dir="ltr"><div><div>Dear Martina,<br><br></div>It might help to distinguish two aspects of cluster-based statistic.<br></div><div><br>1) the statistical approuch that you will use to determine whether a time-channel-datapoint / time-frequency-channel-datapoint / time-frequency-voxel-datapoint is considered significant different between conditions.<br></div><div>2) the statistical approuch that you will use to determine whether <b>a cluster of</b> time-channel-datapoints / time-frequency-channel-datapoints / time-frequency-voxel-datapoints is considered significantly different between conditions.</div><div><br></div><div>When you talk about <b>cluster statistics</b>, you probably think about the second part. But this might not be what you should initially be concerned with when thinking about e.g. different numbers of trials between conditions. Rather, consider what statistical tests you (can) use to compare your time-frequency values between conditions <i>(within subjects).</i> This can be, e.g., a t-test, a nonparametric (e.g. montecarlo) test, or any test, for that matter. As far as my limited knowledge of statistics goes, in most simple and non-extreme cases, unequal number of trials that does not have to increase your chance of type I errors, rather that of type 2 (you'll be insensitive to differences if you don't have enough observations in one condition due to noisy estimate of means/distribution). But in any case it's a simple question to google or ask a statistician.<br><br></div><div>Now, after you are happy with and confident about the between conditions statistical test, consider how the cluster statistics might help you. <br>First of all, how does it determine whether a cluster is significantly different between conditions? There are different options, but the gist is that it takes your significant statistical numbers of step (1), adds them up when they belong to the same cluster (based on whether they are neigbourings in time/freq/space with other significant numbers), takes the maximum of these summed up clusters (there might be more than one cluster), and then compares this one value to the same taken from a<b>(non-parametric) monte-carlo distribution </b>of the null hypothesis based on permuting the values over conditions (and then calculating the maximum sum). The Maris and Oostenveld paper explains it in more detail. <br><br>The reason for doing cluster-statistics is that its a smart way of dealing with multiple comparisons over many time x frequency x channels (or space). The method is blind for your decisions about how its computed for each point in time x frequency x channels (or space). <br><br>I find the FieldTrip statistics functions, their configurations etc., and the way they interact confusing at times, but I hope this helps to clear it up a bit. <br><br></div><div>Long story short - I think your question does not limit itself to cluster statistics and at the same time is much simpler. It's all about (1). <br><br></div><div>Best wishes,<br></div><div>Stephen<br></div><div><br><br><br></div><div><br><br><br><br></div><div><br><br><br><br></div><div><br><br><br></div><div><br>There are two separate steps cluster statistics (as implemented in FieldTrip, but in general as well).<br><br><br></div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On 18 March 2015 at 14:51, Joram van Driel <span dir="ltr"><<a href="mailto:joramvandriel@gmail.com" target="_blank">joramvandriel@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi Martina,<div><br></div><div>In general, I'd advice to do some kind of trial-selection procedure when comparing error versus correct trials, in order to trial-count-match the two conditions. Otherwise you run into problems considering: SNR (higher for the correct condition), and RT (errors are usually faster, resulting in a time-on-task confound). What I always do is pick from the correct condition a similar number of trials that are close to the RT distribution of the error trials (i.e. the faster correct trials). That way you solve both problems at once (and probably the cluster-based permutation test in field trip will work as well, as a bonus ;)).</div><div><br></div><div>Best,</div><div>Joram</div></div><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="h5">On Wed, Mar 18, 2015 at 2:31 PM, Martina Rossi <span dir="ltr"><<a href="mailto:martina.rossi76@yahoo.it" target="_blank">martina.rossi76@yahoo.it</a>></span> wrote:<br></div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5"><div><div style="color:#000;background-color:#fff;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif;font-size:16px"><div><div style="color:#000;background-color:#fff;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif;font-size:16px"><div dir="ltr">Dear All,</div><div dir="ltr"><br></div><div dir="ltr">I would like to get some feedback from the community about a statistical analysis problem I need to tackle with my study.</div><div dir="ltr">I want to apply the cluster-based permutation tests on time-frequency data considering two conditions (correct vs error).</div><div dir="ltr">Unfortunately, these two conditions have different sizes (correct >> error).</div><div dir="ltr">Right
now, I am only considering subjects having a ratio "error/correct"
bigger than 1/5, yet this is only an arbitrary threshold I set.</div><div dir="ltr">The question is the following:</div><div dir="ltr">is
there a formal way to identify a threshold by which two conditions can
be realiably compared with the cluster-based permutation tests?</div><div dir="ltr">If the cluster-based approach is not suitable in this scenario, is there any other approach you would suggest?</div><div dir="ltr">I shall perhaps point out that I am working on EEG data recorded with a 32 channel system (impedance levels < 10 k<span>Ω).</span></div><div dir="ltr"><br></div><div dir="ltr">Looking forward to hear your feedback :)</div><div dir="ltr"><br></div><div dir="ltr">Kind Regards,</div>Martina Rossi<div dir="ltr"><br></div></div></div></div></div><br></div></div>_______________________________________________<br>
fieldtrip mailing list<br>
<a href="mailto:fieldtrip@donders.ru.nl" target="_blank">fieldtrip@donders.ru.nl</a><br>
<a href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target="_blank">http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a><span class="HOEnZb"><font color="#888888"><br></font></span></blockquote></div><span class="HOEnZb"><font color="#888888"><br><br clear="all"><div><br></div>-- <br><div><div dir="ltr"><div><div dir="ltr">Joram van Driel<div>Postdoc @ Vrije Universiteit Amsterdam</div><div>Cognitive Psychology</div><div><br></div></div></div></div></div>
</font></span></div>
<br>_______________________________________________<br>
fieldtrip mailing list<br>
<a href="mailto:fieldtrip@donders.ru.nl">fieldtrip@donders.ru.nl</a><br>
<a href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target="_blank">http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a><br></blockquote></div><br></div>