The Jargon File (version 4.4.7, 29 Dec 2003):

Breidbart Index
 /bri:d'bart ind@ks/

    A measurement of the severity of spam invented by long-time hacker Seth
    Breidbart, used for programming cancelbots. The Breidbart Index takes into
    account the fact that excessive multi-posting EMP is worse than excessive
    cross-posting ECP. The Breidbart Index is computed as follows: For each
    article in a spam, take the square-root of the number of newsgroups to
    which the article is posted. The Breidbart Index is the sum of the square
    roots of all of the posts in the spam. For example, one article posted to
    nine newsgroups and again to sixteen would have BI = sqrt(9) + sqrt(16) =
    7. It is generally agreed that a spam is cancelable if the Breidbart Index
    exceeds 20.

    The Breidbart Index accumulates over a 45-day window. Ten articles
    yesterday and ten articles today and ten articles tomorrow add up to a
    30-article spam. Spam fighters will often reset the count if you can
    convince them that the spam was accidental and/or you have seen the error
    of your ways and won't repeat it. Breidbart Index can accumulate over
    multiple authors. For example, the ?Make Money Fast? pyramid scheme
    exceeded a BI of 20 a long time ago, and is now considered ?cancel on sight
    ?.