Vegetation cluster testing using MRPP

Curt Seeliger cws1 at dirt.first!
Wed May 20 11:47:51 EST 1998


Thanks for the reply Warren.  I'm making hierarchical clusters based on
euclidian distances and linking using Ward's method, so I don't think I'm
warping my space.  I think my transformations of the raw matrix are also
reasonable.  On the otherhand, I'm unversed in the triangle inequality you
mentioned, so I can't comment on this. 

I did get a chance to run the question by a local statistician, who
thought that the zeros in my data (about 87% of which is zero) is causing
the test statistic to degrade i.e. takes on only a few values, so that
only extreme cases turn out to be significant.  An example of this she
gave was testing on a binomial distribution with only 4 points - in this
case only 1 outcome is 'significant'.  Unfortunately, she couldn't
recomend a better means of testing veg clusters, stating the well known
truth that clustering is an art, and generally not subjected to testing.

cur

On Tue, 19 May 1998, Warren Sarle wrote:

> 
> If you are using the same distance matrix for the cluster analysis
> and the MRPP, and your data set is not tiny, then you should indeed
> get wildly "significant" results. I would be surprised to see a
> p-value over .0001. If you are using different distance matrices,
> anything could happen. Perhaps weird things could also happen if
> you used an extremely non-Euclidean distance matrix; for example,
> something that had gross violations of the triangle inequality.
> 







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