Dear all,

I am now working on the hierarchical clustering methods, and
confused about the following problem.

As you know, to form clustering from the hierarchical tree generated by
the pairwise distance bw the elements, we have to set a threshold value
to cut the tree horizonally such that the vertical links intersecting with
this horizonal critical value will be the final clusters.

However, I do not find a very robust criterion for choosing the
optimal number of clusters or calculating this threshold value to make the 
clustering results good different pairwise distance(similairty) measure. 

So any one has some point on this problem or recommended papers
or methods?

Thanks for your help.

Fred