Index of Coincidence

I'm new to the mailing list, but I have grep'd tcmay's CP-FAQ and all related FAQs for this, but have yet to find anything. I'm looking for detailed information on a statistical tool to aid in cryptanalysis called an "index of coincidence". I would appreciate it if someone could please explain what this is, how one builds such an index against a given ciphertext, and how it is valuable in the cryptanalysis of said ciphertext. Many thanks, Marc -- .--------------------------------------------------------. | Marc North <*> mnorth@netcom.com <*> San Jose, CA. | `--------------------------------------------------------'