clusterrandanalysis actvsblT
Eric Maris
maris at NICI.RU.NL
Mon Jul 30 22:49:43 CEST 2007
Dear Claudia,
> i have a questions about clusteranalysis comparing baseline and
> activation period. is it really impossible to compare two periods of
> different length? and why? as far as i understand the mean of the
> baseline is used and so you have only one value per frequency anyway.
> can i avoid this problem or do i have to divide my activation period
> into different segments because my baseline period is much shorter?
You are one of many to complain about this feature of clusterrandanalysis.
Nevertheless, you do have to divide your activation period into different
segments. The reason for this has to do with the null hypothesis that is
tested when you use the activation-versus-baseline statistic. This null
hypothesis is the following: The data in the baseline period are drawn from
the same probability distribution (over the spatiotemporal domain) as the
data in the activation period. This null hypothesis only makes sense if the
spatiotemporal dimensionality of the activation and the baseline period are
equal.
Kind regards,
Eric Maris
dr. Eric Maris
NICI/Biological Psychology and
F.C. Donders Center for Cognitive NeuroImaging
University of Nijmegen
P.O. Box 9104
6500 HE Nijmegen
The Netherlands
T:+31 24 3612651 (NICI)
T:+31 24 3610754 (FCDC)
F:+31 24 3616066 (NICI)
E: maris at nici.ru.nl
MSc Cognitive Neuroscience :www.ru.nl/master/cns/
> thanks in advance
> claudia
>
>
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