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What you have explained here is the Jaccard index (
http://en.wikipedia.org/wiki/Jaccard_index), which is very closely related
to the cosine similarity measure (
http://en.wikipedia.org/wiki/Cosine_similarity). Both measures are used for
measuring how similar two samples (sets, clusters, etc) are.

Best
Aziz

Abedelaziz Mohaisen
Ph.D. Student, UMN
http://www.cs.umn.edu/~mohaisen

> ***** To join INSNA, visit http://www.insna.org ***** A few months ago I
> stumbled across a simple measure of cluster overlap. Assume two clusters, A
> and B, that contain a total of n nodes of which m nodes belong to both
> clusters or, in other words, n is the number of nodes in the union of A and
> B and m the number of nodes in the intersection of A and B. The measure is
> something along the lines of  overlap=m/n=nodes in intersection/nodes in
> union. My problem is (1) that I have forgotten where I found this measure
> and (2) am not 100% sure that it was as simple as I recall. If you recognize
> this problem, can point me to a source or provide me with a correct or
> better formula, your help will be much appreciated.
>
> John
>
> --
> John McCreery
> The Word Works, Ltd., Yokohama, JAPAN
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