about cluster randomization analysis

[Robert Oostenveld]

Hi Marco,

On 8-nov-2005, at 12:25, Marco Buiatti wrote:
> Do you think it would be correct to slide a relatively large (width
> of 200ms? 400ms? to be chosen a priori of course) window through
> the epochs and compute cluster randomization analysis for each
> latency to explore dubious significant t-test clusters?

You can use such an approach, but then you have to consider each
position of the window that you are sliding as a seperate statistical
comparison of the data in the experimental conditions. The multiple
comparison problem over channels and timepoints within the window is
then automatically taken care of by clusterrandanalysis, but the
multiple comparisons that arise due to the multiple locations of the
window in which you are "interrogating" your data are not treated by
clusterrandanalysis. That means that, for this approach to be
statistically completely sound, you should do a Bonferoni correction
on the alpha threshold, dividing it by the number of window positions.

Probably you will loose a lot of your statistical power especially if
you slide the window in small steps, so I doubt whether it is
usefull. Given that you have expressed your doubts about potential
artifacts in some of your subjects and the influence of the artifacts
on the outcome of the statistical test, I would guess that putting
more effort into making the data itself cleaner is probably more
worthwile.

best regards,
Robert


=======================================================
Robert Oostenveld, PhD
F.C. Donders Centre for Cognitive Neuroimaging
Radboud University Nijmegen
phone: +31-24-3619695
http://www.ru.nl/fcdonders/