Problem with data from BESA

Eric Maris maris at NICI.RU.NL
Mon Nov 21 15:59:10 CET 2005

Hi Michael,

> I have however another question regarding the interpretation of
> clusteranalysis results. Am I correct in saying that the family wise error
> rate (alpha) tells me the risk in obtaining a false positive statement of
> the type that I specify previously with alphatresh? For example if I
> specify alphathresh of 0.1 (lets calls this trend for abbreviation) in the
> first pass of the analysis (multiple testing) before clustering then the
> clusterrandomization using alpha =0.05 tells me that I run a risk of 5% of
> identifying wrongly at least one of these 'trend clusters'.
> (Or else, if the above is incorrect what is the reason not to use a very
> lenient criterion in the first pass to feed the clusterrandomization with
> as many clusters as possible?)

The issue is statistical power (sensitivity). If you use a very lenient
criterion (say, alphathresh=0.2) to select candidate cluster members, this
will result in large clusters purely by chance. If the effect in your data
is strong but confined to a small number of sensors and timepoints,
clusterrandanalysis may not pick it up. This is because the reference
distribution is dominated by these weak but large "chance clusters". You
will not encounter this problem if you put alphathresh higher. On the other
hand, a high aphathresh will miss weak but widespread effects.

To sum up, alphathresh determines the relative sensitivity to "strong but
small" and "weak but large" clusters.


Eric Maris

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