Statistically Improbable Phrases

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Statistically Improbable Phrases or SIPs is a system developed by Amazon.com to compare all of the books they index in the Search Inside! program and find phrases in each that are the most unlikely to be found in any other book indexed.

The system is used to find the most nearly unique portions of books for use as a summary or keyword.

[edit] Example

Book 1

The big brown fox jumps over the lazy dog. The lazy dog did not like the fact that the big brown fox jumped over him, so the lazy dog ran after him.

Book 2

You never have to log in to read Wikipedia. You do not have to log in even to edit articles on Wikipedia—anyone can edit almost any article, even without logging in. Nevertheless, creating an account is quick, free and non-intrusive, and it's generally considered a good idea to do so, for a variety of reasons.

Book 3

If you create an account, you can pick a username. Edits you make while logged in will be assigned to that name. That means you get full credit for your contributions in the page history (when not logged in, the edits are just assigned to your (potentially random) IP address). You can also view all your contributions by clicking the "My contributions" link, which is only visible when you are logged in.

SIPs

For Book 1, the SIP would most likely be "Big Brown Fox" and "Lazy Dog"

For Book 2, the SIP would most likely be "Wikipedia", but not "account" because it is featured in Book 3 many times.

For Book 3, the SIP would most likely be "Contributions", and "Logged In"