R help - Apr 2004 - How to get the threshold value for hierarchical clustering algorithms?

Dear R users,

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 are the clusters.

However, I do not find any theorectical results or papers discussing
how to calculate this threshold value to make the clustering results
more reliable.

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

Thanks for your help.

Fred
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