Breidbart Index
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The Breidbart Index, developed by Seth Breidbart, provides a measure of severity of newsgroup spam. The Breidbart Index is calculated over a 45-day window, and takes into account the number of newsgroups a message is posted to. It is defined as the sum over each copy of the message of the square root of the number of newsgroups that copy is crossposted to. Articles are considered the same if they are substantively identical.
For the Big 8 and alt.* hierarchies, it's generally agreed that messages are cancellable spam when the Breidbart Index exceeds 20, at which point they can be auto-cancelled from news servers. Other hierarchies have their own rules; many (smaller, local ones) are much more restrictive.de:Breidbart-Index